diff --git a/google/auth/_default.py b/google/auth/_default.py
index 3b8c281..8e3b210 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -34,7 +34,8 @@
 # Valid types accepted for file-based credentials.
 _AUTHORIZED_USER_TYPE = "authorized_user"
 _SERVICE_ACCOUNT_TYPE = "service_account"
-_VALID_TYPES = (_AUTHORIZED_USER_TYPE, _SERVICE_ACCOUNT_TYPE)
+_EXTERNAL_ACCOUNT_TYPE = "external_account"
+_VALID_TYPES = (_AUTHORIZED_USER_TYPE, _SERVICE_ACCOUNT_TYPE, _EXTERNAL_ACCOUNT_TYPE)
 
 # Help message when no credentials can be found.
 _HELP_MESSAGE = """\
@@ -70,12 +71,12 @@
 
 
 def load_credentials_from_file(
-    filename, scopes=None, default_scopes=None, quota_project_id=None
+    filename, scopes=None, default_scopes=None, quota_project_id=None, request=None
 ):
     """Loads Google credentials from a file.
 
-    The credentials file must be a service account key or stored authorized
-    user credentials.
+    The credentials file must be a service account key, stored authorized
+    user credentials or external account credentials.
 
     Args:
         filename (str): The full path to the credentials file.
@@ -85,12 +86,18 @@
         default_scopes (Optional[Sequence[str]]): Default scopes passed by a
             Google client library. Use 'scopes' for user-defined scopes.
         quota_project_id (Optional[str]):  The project ID used for
-                quota and billing.
+            quota and billing.
+        request (Optional[google.auth.transport.Request]): An object used to make
+            HTTP requests. This is used to determine the associated project ID
+            for a workload identity pool resource (external account credentials).
+            If not specified, then it will use a
+            google.auth.transport.requests.Request client to make requests.
 
     Returns:
         Tuple[google.auth.credentials.Credentials, Optional[str]]: Loaded
             credentials and the project ID. Authorized user credentials do not
-            have the project ID information.
+            have the project ID information. External account credentials project
+            IDs may not always be determined.
 
     Raises:
         google.auth.exceptions.DefaultCredentialsError: if the file is in the
@@ -146,6 +153,18 @@
             credentials = credentials.with_quota_project(quota_project_id)
         return credentials, info.get("project_id")
 
+    elif credential_type == _EXTERNAL_ACCOUNT_TYPE:
+        credentials, project_id = _get_external_account_credentials(
+            info,
+            filename,
+            scopes=scopes,
+            default_scopes=default_scopes,
+            request=request,
+        )
+        if quota_project_id:
+            credentials = credentials.with_quota_project(quota_project_id)
+        return credentials, project_id
+
     else:
         raise exceptions.DefaultCredentialsError(
             "The file {file} does not have a valid type. "
@@ -252,6 +271,65 @@
         return None, None
 
 
+def _get_external_account_credentials(
+    info, filename, scopes=None, default_scopes=None, request=None
+):
+    """Loads external account Credentials from the parsed external account info.
+
+    The credentials information must correspond to a supported external account
+    credentials.
+
+    Args:
+        info (Mapping[str, str]): The external account info in Google format.
+        filename (str): The full path to the credentials file.
+        scopes (Optional[Sequence[str]]): The list of scopes for the credentials. If
+            specified, the credentials will automatically be scoped if
+            necessary.
+        default_scopes (Optional[Sequence[str]]): Default scopes passed by a
+            Google client library. Use 'scopes' for user-defined scopes.
+        request (Optional[google.auth.transport.Request]): An object used to make
+            HTTP requests. This is used to determine the associated project ID
+            for a workload identity pool resource (external account credentials).
+            If not specified, then it will use a
+            google.auth.transport.requests.Request client to make requests.
+
+    Returns:
+        Tuple[google.auth.credentials.Credentials, Optional[str]]: Loaded
+            credentials and the project ID. External account credentials project
+            IDs may not always be determined.
+
+    Raises:
+        google.auth.exceptions.DefaultCredentialsError: if the info dictionary
+            is in the wrong format or is missing required information.
+    """
+    # There are currently 2 types of external_account credentials.
+    try:
+        # Check if configuration corresponds to an AWS credentials.
+        from google.auth import aws
+
+        credentials = aws.Credentials.from_info(
+            info, scopes=scopes, default_scopes=default_scopes
+        )
+    except ValueError:
+        try:
+            # Check if configuration corresponds to an Identity Pool credentials.
+            from google.auth import identity_pool
+
+            credentials = identity_pool.Credentials.from_info(
+                info, scopes=scopes, default_scopes=default_scopes
+            )
+        except ValueError:
+            # If the configuration is invalid or does not correspond to any
+            # supported external_account credentials, raise an error.
+            raise exceptions.DefaultCredentialsError(
+                "Failed to load external account credentials from {}".format(filename)
+            )
+    if request is None:
+        request = google.auth.transport.requests.Request()
+
+    return credentials, credentials.get_project_id(request=request)
+
+
 def default(scopes=None, request=None, quota_project_id=None, default_scopes=None):
     """Gets the default credentials for the current environment.
 
@@ -265,6 +343,15 @@
        loaded and returned. The project ID returned is the project ID defined
        in the service account file if available (some older files do not
        contain project ID information).
+
+       If the environment variable is set to the path of a valid external
+       account JSON configuration file (workload identity federation), then the
+       configuration file is used to determine and retrieve the external
+       credentials from the current environment (AWS, Azure, etc).
+       These will then be exchanged for Google access tokens via the Google STS
+       endpoint.
+       The project ID returned in this case is the one corresponding to the
+       underlying workload identity pool resource if determinable.
     2. If the `Google Cloud SDK`_ is installed and has application default
        credentials set they are loaded and returned.
 
@@ -310,11 +397,15 @@
         scopes (Sequence[str]): The list of scopes for the credentials. If
             specified, the credentials will automatically be scoped if
             necessary.
-        request (google.auth.transport.Request): An object used to make
-            HTTP requests. This is used to detect whether the application
-            is running on Compute Engine. If not specified, then it will
-            use the standard library http client to make requests.
-        quota_project_id (Optional[str]):  The project ID used for
+        request (Optional[google.auth.transport.Request]): An object used to make
+            HTTP requests. This is used to either detect whether the application
+            is running on Compute Engine or to determine the associated project
+            ID for a workload identity pool resource (external account
+            credentials). If not specified, then it will either use the standard
+            library http client to make requests for Compute Engine credentials
+            or a google.auth.transport.requests.Request client for external
+            account credentials.
+        quota_project_id (Optional[str]): The project ID used for
             quota and billing.
         default_scopes (Optional[Sequence[str]]): Default scopes passed by a
             Google client library. Use 'scopes' for user-defined scopes.
@@ -336,6 +427,10 @@
     )
 
     checkers = (
+        # Avoid passing scopes here to prevent passing scopes to user credentials.
+        # with_scopes_if_required() below will ensure scopes/default scopes are
+        # safely set on the returned credentials since requires_scopes will
+        # guard against setting scopes on user credentials.
         _get_explicit_environ_credentials,
         _get_gcloud_sdk_credentials,
         _get_gae_credentials,
@@ -348,6 +443,17 @@
             credentials = with_scopes_if_required(
                 credentials, scopes, default_scopes=default_scopes
             )
+
+            # For external account credentials, scopes are required to determine
+            # the project ID. Try to get the project ID again if not yet
+            # determined.
+            if not project_id and callable(
+                getattr(credentials, "get_project_id", None)
+            ):
+                if request is None:
+                    request = google.auth.transport.requests.Request()
+                project_id = credentials.get_project_id(request=request)
+
             if quota_project_id:
                 credentials = credentials.with_quota_project(quota_project_id)
 
diff --git a/google/auth/aws.py b/google/auth/aws.py
new file mode 100644
index 0000000..b362dd3
--- /dev/null
+++ b/google/auth/aws.py
@@ -0,0 +1,714 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""AWS Credentials and AWS Signature V4 Request Signer.
+
+This module provides credentials to access Google Cloud resources from Amazon
+Web Services (AWS) workloads. These credentials are recommended over the
+use of service account credentials in AWS as they do not involve the management
+of long-live service account private keys.
+
+AWS Credentials are initialized using external_account arguments which are
+typically loaded from the external credentials JSON file.
+Unlike other Credentials that can be initialized with a list of explicit
+arguments, secrets or credentials, external account clients use the
+environment and hints/guidelines provided by the external_account JSON
+file to retrieve credentials and exchange them for Google access tokens.
+
+This module also provides a basic implementation of the
+`AWS Signature Version 4`_ request signing algorithm.
+
+AWS Credentials use serialized signed requests to the
+`AWS STS GetCallerIdentity`_ API that can be exchanged for Google access tokens
+via the GCP STS endpoint.
+
+.. _AWS Signature Version 4: https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html
+.. _AWS STS GetCallerIdentity: https://docs.aws.amazon.com/STS/latest/APIReference/API_GetCallerIdentity.html
+"""
+
+import hashlib
+import hmac
+import io
+import json
+import os
+import re
+
+from six.moves import http_client
+from six.moves import urllib
+
+from google.auth import _helpers
+from google.auth import environment_vars
+from google.auth import exceptions
+from google.auth import external_account
+
+# AWS Signature Version 4 signing algorithm identifier.
+_AWS_ALGORITHM = "AWS4-HMAC-SHA256"
+# The termination string for the AWS credential scope value as defined in
+# https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html
+_AWS_REQUEST_TYPE = "aws4_request"
+# The AWS authorization header name for the security session token if available.
+_AWS_SECURITY_TOKEN_HEADER = "x-amz-security-token"
+# The AWS authorization header name for the auto-generated date.
+_AWS_DATE_HEADER = "x-amz-date"
+
+
+class RequestSigner(object):
+    """Implements an AWS request signer based on the AWS Signature Version 4 signing
+    process.
+    https://docs.aws.amazon.com/general/latest/gr/signature-version-4.html
+    """
+
+    def __init__(self, region_name):
+        """Instantiates an AWS request signer used to compute authenticated signed
+        requests to AWS APIs based on the AWS Signature Version 4 signing process.
+
+        Args:
+            region_name (str): The AWS region to use.
+        """
+
+        self._region_name = region_name
+
+    def get_request_options(
+        self,
+        aws_security_credentials,
+        url,
+        method,
+        request_payload="",
+        additional_headers={},
+    ):
+        """Generates the signed request for the provided HTTP request for calling
+        an AWS API. This follows the steps described at:
+        https://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html
+
+        Args:
+            aws_security_credentials (Mapping[str, str]): A dictionary containing
+                the AWS security credentials.
+            url (str): The AWS service URL containing the canonical URI and
+                query string.
+            method (str): The HTTP method used to call this API.
+            request_payload (Optional[str]): The optional request payload if
+                available.
+            additional_headers (Optional[Mapping[str, str]]): The optional
+                additional headers needed for the requested AWS API.
+
+        Returns:
+            Mapping[str, str]: The AWS signed request dictionary object.
+        """
+        # Get AWS credentials.
+        access_key = aws_security_credentials.get("access_key_id")
+        secret_key = aws_security_credentials.get("secret_access_key")
+        security_token = aws_security_credentials.get("security_token")
+
+        additional_headers = additional_headers or {}
+
+        uri = urllib.parse.urlparse(url)
+        # Validate provided URL.
+        if not uri.hostname or uri.scheme != "https":
+            raise ValueError("Invalid AWS service URL")
+
+        header_map = _generate_authentication_header_map(
+            host=uri.hostname,
+            canonical_uri=os.path.normpath(uri.path or "/"),
+            canonical_querystring=_get_canonical_querystring(uri.query),
+            method=method,
+            region=self._region_name,
+            access_key=access_key,
+            secret_key=secret_key,
+            security_token=security_token,
+            request_payload=request_payload,
+            additional_headers=additional_headers,
+        )
+        headers = {
+            "Authorization": header_map.get("authorization_header"),
+            "host": uri.hostname,
+        }
+        # Add x-amz-date if available.
+        if "amz_date" in header_map:
+            headers[_AWS_DATE_HEADER] = header_map.get("amz_date")
+        # Append additional optional headers, eg. X-Amz-Target, Content-Type, etc.
+        for key in additional_headers:
+            headers[key] = additional_headers[key]
+
+        # Add session token if available.
+        if security_token is not None:
+            headers[_AWS_SECURITY_TOKEN_HEADER] = security_token
+
+        signed_request = {"url": url, "method": method, "headers": headers}
+        if request_payload:
+            signed_request["data"] = request_payload
+        return signed_request
+
+
+def _get_canonical_querystring(query):
+    """Generates the canonical query string given a raw query string.
+    Logic is based on
+    https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html
+
+    Args:
+        query (str): The raw query string.
+
+    Returns:
+        str: The canonical query string.
+    """
+    # Parse raw query string.
+    querystring = urllib.parse.parse_qs(query)
+    querystring_encoded_map = {}
+    for key in querystring:
+        quote_key = urllib.parse.quote(key, safe="-_.~")
+        # URI encode key.
+        querystring_encoded_map[quote_key] = []
+        for item in querystring[key]:
+            # For each key, URI encode all values for that key.
+            querystring_encoded_map[quote_key].append(
+                urllib.parse.quote(item, safe="-_.~")
+            )
+        # Sort values for each key.
+        querystring_encoded_map[quote_key].sort()
+    # Sort keys.
+    sorted_keys = list(querystring_encoded_map.keys())
+    sorted_keys.sort()
+    # Reconstruct the query string. Preserve keys with multiple values.
+    querystring_encoded_pairs = []
+    for key in sorted_keys:
+        for item in querystring_encoded_map[key]:
+            querystring_encoded_pairs.append("{}={}".format(key, item))
+    return "&".join(querystring_encoded_pairs)
+
+
+def _sign(key, msg):
+    """Creates the HMAC-SHA256 hash of the provided message using the provided
+    key.
+
+    Args:
+        key (str): The HMAC-SHA256 key to use.
+        msg (str): The message to hash.
+
+    Returns:
+        str: The computed hash bytes.
+    """
+    return hmac.new(key, msg.encode("utf-8"), hashlib.sha256).digest()
+
+
+def _get_signing_key(key, date_stamp, region_name, service_name):
+    """Calculates the signing key used to calculate the signature for
+    AWS Signature Version 4 based on:
+    https://docs.aws.amazon.com/general/latest/gr/sigv4-calculate-signature.html
+
+    Args:
+        key (str): The AWS secret access key.
+        date_stamp (str): The '%Y%m%d' date format.
+        region_name (str): The AWS region.
+        service_name (str): The AWS service name, eg. sts.
+
+    Returns:
+        str: The signing key bytes.
+    """
+    k_date = _sign(("AWS4" + key).encode("utf-8"), date_stamp)
+    k_region = _sign(k_date, region_name)
+    k_service = _sign(k_region, service_name)
+    k_signing = _sign(k_service, "aws4_request")
+    return k_signing
+
+
+def _generate_authentication_header_map(
+    host,
+    canonical_uri,
+    canonical_querystring,
+    method,
+    region,
+    access_key,
+    secret_key,
+    security_token,
+    request_payload="",
+    additional_headers={},
+):
+    """Generates the authentication header map needed for generating the AWS
+    Signature Version 4 signed request.
+
+    Args:
+        host (str): The AWS service URL hostname.
+        canonical_uri (str): The AWS service URL path name.
+        canonical_querystring (str): The AWS service URL query string.
+        method (str): The HTTP method used to call this API.
+        region (str): The AWS region.
+        access_key (str): The AWS access key ID.
+        secret_key (str): The AWS secret access key.
+        security_token (Optional[str]): The AWS security session token. This is
+            available for temporary sessions.
+        request_payload (Optional[str]): The optional request payload if
+            available.
+        additional_headers (Optional[Mapping[str, str]]): The optional
+            additional headers needed for the requested AWS API.
+
+    Returns:
+        Mapping[str, str]: The AWS authentication header dictionary object.
+            This contains the x-amz-date and authorization header information.
+    """
+    # iam.amazonaws.com host => iam service.
+    # sts.us-east-2.amazonaws.com host => sts service.
+    service_name = host.split(".")[0]
+
+    current_time = _helpers.utcnow()
+    amz_date = current_time.strftime("%Y%m%dT%H%M%SZ")
+    date_stamp = current_time.strftime("%Y%m%d")
+
+    # Change all additional headers to be lower case.
+    full_headers = {}
+    for key in additional_headers:
+        full_headers[key.lower()] = additional_headers[key]
+    # Add AWS session token if available.
+    if security_token is not None:
+        full_headers[_AWS_SECURITY_TOKEN_HEADER] = security_token
+
+    # Required headers
+    full_headers["host"] = host
+    # Do not use generated x-amz-date if the date header is provided.
+    # Previously the date was not fixed with x-amz- and could be provided
+    # manually.
+    # https://github.com/boto/botocore/blob/879f8440a4e9ace5d3cf145ce8b3d5e5ffb892ef/tests/unit/auth/aws4_testsuite/get-header-value-trim.req
+    if "date" not in full_headers:
+        full_headers[_AWS_DATE_HEADER] = amz_date
+
+    # Header keys need to be sorted alphabetically.
+    canonical_headers = ""
+    header_keys = list(full_headers.keys())
+    header_keys.sort()
+    for key in header_keys:
+        canonical_headers = "{}{}:{}\n".format(
+            canonical_headers, key, full_headers[key]
+        )
+    signed_headers = ";".join(header_keys)
+
+    payload_hash = hashlib.sha256((request_payload or "").encode("utf-8")).hexdigest()
+
+    # https://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html
+    canonical_request = "{}\n{}\n{}\n{}\n{}\n{}".format(
+        method,
+        canonical_uri,
+        canonical_querystring,
+        canonical_headers,
+        signed_headers,
+        payload_hash,
+    )
+
+    credential_scope = "{}/{}/{}/{}".format(
+        date_stamp, region, service_name, _AWS_REQUEST_TYPE
+    )
+
+    # https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html
+    string_to_sign = "{}\n{}\n{}\n{}".format(
+        _AWS_ALGORITHM,
+        amz_date,
+        credential_scope,
+        hashlib.sha256(canonical_request.encode("utf-8")).hexdigest(),
+    )
+
+    # https://docs.aws.amazon.com/general/latest/gr/sigv4-calculate-signature.html
+    signing_key = _get_signing_key(secret_key, date_stamp, region, service_name)
+    signature = hmac.new(
+        signing_key, string_to_sign.encode("utf-8"), hashlib.sha256
+    ).hexdigest()
+
+    # https://docs.aws.amazon.com/general/latest/gr/sigv4-add-signature-to-request.html
+    authorization_header = "{} Credential={}/{}, SignedHeaders={}, Signature={}".format(
+        _AWS_ALGORITHM, access_key, credential_scope, signed_headers, signature
+    )
+
+    authentication_header = {"authorization_header": authorization_header}
+    # Do not use generated x-amz-date if the date header is provided.
+    if "date" not in full_headers:
+        authentication_header["amz_date"] = amz_date
+    return authentication_header
+
+
+class Credentials(external_account.Credentials):
+    """AWS external account credentials.
+    This is used to exchange serialized AWS signature v4 signed requests to
+    AWS STS GetCallerIdentity service for Google access tokens.
+    """
+
+    def __init__(
+        self,
+        audience,
+        subject_token_type,
+        token_url,
+        credential_source=None,
+        service_account_impersonation_url=None,
+        client_id=None,
+        client_secret=None,
+        quota_project_id=None,
+        scopes=None,
+        default_scopes=None,
+    ):
+        """Instantiates an AWS workload external account credentials object.
+
+        Args:
+            audience (str): The STS audience field.
+            subject_token_type (str): The subject token type.
+            token_url (str): The STS endpoint URL.
+            credential_source (Mapping): The credential source dictionary used
+                to provide instructions on how to retrieve external credential
+                to be exchanged for Google access tokens.
+            service_account_impersonation_url (Optional[str]): The optional
+                service account impersonation getAccessToken URL.
+            client_id (Optional[str]): The optional client ID.
+            client_secret (Optional[str]): The optional client secret.
+            quota_project_id (Optional[str]): The optional quota project ID.
+            scopes (Optional[Sequence[str]]): Optional scopes to request during
+                the authorization grant.
+            default_scopes (Optional[Sequence[str]]): Default scopes passed by a
+                Google client library. Use 'scopes' for user-defined scopes.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error is encountered during
+                access token retrieval logic.
+            ValueError: For invalid parameters.
+
+        .. note:: Typically one of the helper constructors
+            :meth:`from_file` or
+            :meth:`from_info` are used instead of calling the constructor directly.
+        """
+        super(Credentials, self).__init__(
+            audience=audience,
+            subject_token_type=subject_token_type,
+            token_url=token_url,
+            credential_source=credential_source,
+            service_account_impersonation_url=service_account_impersonation_url,
+            client_id=client_id,
+            client_secret=client_secret,
+            quota_project_id=quota_project_id,
+            scopes=scopes,
+            default_scopes=default_scopes,
+        )
+        credential_source = credential_source or {}
+        self._environment_id = credential_source.get("environment_id") or ""
+        self._region_url = credential_source.get("region_url")
+        self._security_credentials_url = credential_source.get("url")
+        self._cred_verification_url = credential_source.get(
+            "regional_cred_verification_url"
+        )
+        self._region = None
+        self._request_signer = None
+        self._target_resource = audience
+
+        # Get the environment ID. Currently, only one version supported (v1).
+        matches = re.match(r"^(aws)([\d]+)$", self._environment_id)
+        if matches:
+            env_id, env_version = matches.groups()
+        else:
+            env_id, env_version = (None, None)
+
+        if env_id != "aws" or self._cred_verification_url is None:
+            raise ValueError("No valid AWS 'credential_source' provided")
+        elif int(env_version or "") != 1:
+            raise ValueError(
+                "aws version '{}' is not supported in the current build.".format(
+                    env_version
+                )
+            )
+
+    def retrieve_subject_token(self, request):
+        """Retrieves the subject token using the credential_source object.
+        The subject token is a serialized `AWS GetCallerIdentity signed request`_.
+
+        The logic is summarized as:
+
+        Retrieve the AWS region from the AWS_REGION environment variable or from
+        the AWS metadata server availability-zone if not found in the
+        environment variable.
+
+        Check AWS credentials in environment variables. If not found, retrieve
+        from the AWS metadata server security-credentials endpoint.
+
+        When retrieving AWS credentials from the metadata server
+        security-credentials endpoint, the AWS role needs to be determined by
+        calling the security-credentials endpoint without any argument. Then the
+        credentials can be retrieved via: security-credentials/role_name
+
+        Generate the signed request to AWS STS GetCallerIdentity action.
+
+        Inject x-goog-cloud-target-resource into header and serialize the
+        signed request. This will be the subject-token to pass to GCP STS.
+
+        .. _AWS GetCallerIdentity signed request:
+            https://cloud.google.com/iam/docs/access-resources-aws#exchange-token
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+        Returns:
+            str: The retrieved subject token.
+        """
+        # Initialize the request signer if not yet initialized after determining
+        # the current AWS region.
+        if self._request_signer is None:
+            self._region = self._get_region(request, self._region_url)
+            self._request_signer = RequestSigner(self._region)
+
+        # Retrieve the AWS security credentials needed to generate the signed
+        # request.
+        aws_security_credentials = self._get_security_credentials(request)
+        # Generate the signed request to AWS STS GetCallerIdentity API.
+        # Use the required regional endpoint. Otherwise, the request will fail.
+        request_options = self._request_signer.get_request_options(
+            aws_security_credentials,
+            self._cred_verification_url.replace("{region}", self._region),
+            "POST",
+        )
+        # The GCP STS endpoint expects the headers to be formatted as:
+        # [
+        #   {key: 'x-amz-date', value: '...'},
+        #   {key: 'Authorization', value: '...'},
+        #   ...
+        # ]
+        # And then serialized as:
+        # quote(json.dumps({
+        #   url: '...',
+        #   method: 'POST',
+        #   headers: [{key: 'x-amz-date', value: '...'}, ...]
+        # }))
+        request_headers = request_options.get("headers")
+        # The full, canonical resource name of the workload identity pool
+        # provider, with or without the HTTPS prefix.
+        # Including this header as part of the signature is recommended to
+        # ensure data integrity.
+        request_headers["x-goog-cloud-target-resource"] = self._target_resource
+
+        # Serialize AWS signed request.
+        # Keeping inner keys in sorted order makes testing easier for Python
+        # versions <=3.5 as the stringified JSON string would have a predictable
+        # key order.
+        aws_signed_req = {}
+        aws_signed_req["url"] = request_options.get("url")
+        aws_signed_req["method"] = request_options.get("method")
+        aws_signed_req["headers"] = []
+        # Reformat header to GCP STS expected format.
+        for key in sorted(request_headers.keys()):
+            aws_signed_req["headers"].append(
+                {"key": key, "value": request_headers[key]}
+            )
+
+        return urllib.parse.quote(
+            json.dumps(aws_signed_req, separators=(",", ":"), sort_keys=True)
+        )
+
+    def _get_region(self, request, url):
+        """Retrieves the current AWS region from either the AWS_REGION
+        environment variable or from the AWS metadata server.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+            url (str): The AWS metadata server region URL.
+
+        Returns:
+            str: The current AWS region.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error occurs while
+                retrieving the AWS region.
+        """
+        # The AWS metadata server is not available in some AWS environments
+        # such as AWS lambda. Instead, it is available via environment
+        # variable.
+        env_aws_region = os.environ.get(environment_vars.AWS_REGION)
+        if env_aws_region is not None:
+            return env_aws_region
+
+        if not self._region_url:
+            raise exceptions.RefreshError("Unable to determine AWS region")
+        response = request(url=self._region_url, method="GET")
+
+        # Support both string and bytes type response.data.
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
+
+        if response.status != 200:
+            raise exceptions.RefreshError(
+                "Unable to retrieve AWS region", response_body
+            )
+
+        # This endpoint will return the region in format: us-east-2b.
+        # Only the us-east-2 part should be used.
+        return response_body[:-1]
+
+    def _get_security_credentials(self, request):
+        """Retrieves the AWS security credentials required for signing AWS
+        requests from either the AWS security credentials environment variables
+        or from the AWS metadata server.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+
+        Returns:
+            Mapping[str, str]: The AWS security credentials dictionary object.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error occurs while
+                retrieving the AWS security credentials.
+        """
+
+        # Check environment variables for permanent credentials first.
+        # https://docs.aws.amazon.com/general/latest/gr/aws-sec-cred-types.html
+        env_aws_access_key_id = os.environ.get(environment_vars.AWS_ACCESS_KEY_ID)
+        env_aws_secret_access_key = os.environ.get(
+            environment_vars.AWS_SECRET_ACCESS_KEY
+        )
+        # This is normally not available for permanent credentials.
+        env_aws_session_token = os.environ.get(environment_vars.AWS_SESSION_TOKEN)
+        if env_aws_access_key_id and env_aws_secret_access_key:
+            return {
+                "access_key_id": env_aws_access_key_id,
+                "secret_access_key": env_aws_secret_access_key,
+                "security_token": env_aws_session_token,
+            }
+
+        # Get role name.
+        role_name = self._get_metadata_role_name(request)
+
+        # Get security credentials.
+        credentials = self._get_metadata_security_credentials(request, role_name)
+
+        return {
+            "access_key_id": credentials.get("AccessKeyId"),
+            "secret_access_key": credentials.get("SecretAccessKey"),
+            "security_token": credentials.get("Token"),
+        }
+
+    def _get_metadata_security_credentials(self, request, role_name):
+        """Retrieves the AWS security credentials required for signing AWS
+        requests from the AWS metadata server.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+            role_name (str): The AWS role name required by the AWS metadata
+                server security_credentials endpoint in order to return the
+                credentials.
+
+        Returns:
+            Mapping[str, str]: The AWS metadata server security credentials
+                response.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error occurs while
+                retrieving the AWS security credentials.
+        """
+        headers = {"Content-Type": "application/json"}
+        response = request(
+            url="{}/{}".format(self._security_credentials_url, role_name),
+            method="GET",
+            headers=headers,
+        )
+
+        # support both string and bytes type response.data
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
+
+        if response.status != http_client.OK:
+            raise exceptions.RefreshError(
+                "Unable to retrieve AWS security credentials", response_body
+            )
+
+        credentials_response = json.loads(response_body)
+
+        return credentials_response
+
+    def _get_metadata_role_name(self, request):
+        """Retrieves the AWS role currently attached to the current AWS
+        workload by querying the AWS metadata server. This is needed for the
+        AWS metadata server security credentials endpoint in order to retrieve
+        the AWS security credentials needed to sign requests to AWS APIs.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+
+        Returns:
+            str: The AWS role name.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error occurs while
+                retrieving the AWS role name.
+        """
+        if self._security_credentials_url is None:
+            raise exceptions.RefreshError(
+                "Unable to determine the AWS metadata server security credentials endpoint"
+            )
+        response = request(url=self._security_credentials_url, method="GET")
+
+        # support both string and bytes type response.data
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
+
+        if response.status != http_client.OK:
+            raise exceptions.RefreshError(
+                "Unable to retrieve AWS role name", response_body
+            )
+
+        return response_body
+
+    @classmethod
+    def from_info(cls, info, **kwargs):
+        """Creates an AWS Credentials instance from parsed external account info.
+
+        Args:
+            info (Mapping[str, str]): The AWS external account info in Google
+                format.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.aws.Credentials: The constructed credentials.
+
+        Raises:
+            ValueError: For invalid parameters.
+        """
+        return cls(
+            audience=info.get("audience"),
+            subject_token_type=info.get("subject_token_type"),
+            token_url=info.get("token_url"),
+            service_account_impersonation_url=info.get(
+                "service_account_impersonation_url"
+            ),
+            client_id=info.get("client_id"),
+            client_secret=info.get("client_secret"),
+            credential_source=info.get("credential_source"),
+            quota_project_id=info.get("quota_project_id"),
+            **kwargs
+        )
+
+    @classmethod
+    def from_file(cls, filename, **kwargs):
+        """Creates an AWS Credentials instance from an external account json file.
+
+        Args:
+            filename (str): The path to the AWS external account json file.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.aws.Credentials: The constructed credentials.
+        """
+        with io.open(filename, "r", encoding="utf-8") as json_file:
+            data = json.load(json_file)
+            return cls.from_info(data, **kwargs)
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
index 46a8926..416bab0 100644
--- a/google/auth/environment_vars.py
+++ b/google/auth/environment_vars.py
@@ -59,3 +59,13 @@
 
 The default value is false. Users have to explicitly set this value to true
 in order to use client certificate to establish a mutual TLS channel."""
+
+# AWS environment variables used with AWS workload identity pools to retrieve
+# AWS security credentials and the AWS region needed to create a serialized
+# signed requests to the AWS STS GetCalledIdentity API that can be exchanged
+# for a Google access tokens via the GCP STS endpoint.
+# When not available the AWS metadata server is used to retrieve these values.
+AWS_ACCESS_KEY_ID = "AWS_ACCESS_KEY_ID"
+AWS_SECRET_ACCESS_KEY = "AWS_SECRET_ACCESS_KEY"
+AWS_SESSION_TOKEN = "AWS_SESSION_TOKEN"
+AWS_REGION = "AWS_REGION"
diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
index da06d86..b6f686b 100644
--- a/google/auth/exceptions.py
+++ b/google/auth/exceptions.py
@@ -43,3 +43,8 @@
 
 class ClientCertError(GoogleAuthError):
     """Used to indicate that client certificate is missing or invalid."""
+
+
+class OAuthError(GoogleAuthError):
+    """Used to indicate an error occurred during an OAuth related HTTP
+    request."""
diff --git a/google/auth/external_account.py b/google/auth/external_account.py
new file mode 100644
index 0000000..0429ee0
--- /dev/null
+++ b/google/auth/external_account.py
@@ -0,0 +1,305 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""External Account Credentials.
+
+This module provides credentials that exchange workload identity pool external
+credentials for Google access tokens. This facilitates accessing Google Cloud
+Platform resources from on-prem and non-Google Cloud platforms (e.g. AWS,
+Microsoft Azure, OIDC identity providers), using native credentials retrieved
+from the current environment without the need to copy, save and manage
+long-lived service account credentials.
+
+Specifically, this is intended to use access tokens acquired using the GCP STS
+token exchange endpoint following the `OAuth 2.0 Token Exchange`_ spec.
+
+.. _OAuth 2.0 Token Exchange: https://tools.ietf.org/html/rfc8693
+"""
+
+import abc
+import datetime
+import json
+
+import six
+
+from google.auth import _helpers
+from google.auth import credentials
+from google.auth import exceptions
+from google.auth import impersonated_credentials
+from google.oauth2 import sts
+from google.oauth2 import utils
+
+# The token exchange grant_type used for exchanging credentials.
+_STS_GRANT_TYPE = "urn:ietf:params:oauth:grant-type:token-exchange"
+# The token exchange requested_token_type. This is always an access_token.
+_STS_REQUESTED_TOKEN_TYPE = "urn:ietf:params:oauth:token-type:access_token"
+# Cloud resource manager URL used to retrieve project information.
+_CLOUD_RESOURCE_MANAGER = "https://cloudresourcemanager.googleapis.com/v1/projects/"
+
+
+@six.add_metaclass(abc.ABCMeta)
+class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
+    """Base class for all external account credentials.
+
+    This is used to instantiate Credentials for exchanging external account
+    credentials for Google access token and authorizing requests to Google APIs.
+    The base class implements the common logic for exchanging external account
+    credentials for Google access tokens.
+    """
+
+    def __init__(
+        self,
+        audience,
+        subject_token_type,
+        token_url,
+        credential_source,
+        service_account_impersonation_url=None,
+        client_id=None,
+        client_secret=None,
+        quota_project_id=None,
+        scopes=None,
+        default_scopes=None,
+    ):
+        """Instantiates an external account credentials object.
+
+        Args:
+            audience (str): The STS audience field.
+            subject_token_type (str): The subject token type.
+            token_url (str): The STS endpoint URL.
+            credential_source (Mapping): The credential source dictionary.
+            service_account_impersonation_url (Optional[str]): The optional service account
+                impersonation generateAccessToken URL.
+            client_id (Optional[str]): The optional client ID.
+            client_secret (Optional[str]): The optional client secret.
+            quota_project_id (Optional[str]): The optional quota project ID.
+            scopes (Optional[Sequence[str]]): Optional scopes to request during the
+                authorization grant.
+            default_scopes (Optional[Sequence[str]]): Default scopes passed by a
+                Google client library. Use 'scopes' for user-defined scopes.
+        Raises:
+            google.auth.exceptions.RefreshError: If the generateAccessToken
+                endpoint returned an error.
+        """
+        super(Credentials, self).__init__()
+        self._audience = audience
+        self._subject_token_type = subject_token_type
+        self._token_url = token_url
+        self._credential_source = credential_source
+        self._service_account_impersonation_url = service_account_impersonation_url
+        self._client_id = client_id
+        self._client_secret = client_secret
+        self._quota_project_id = quota_project_id
+        self._scopes = scopes
+        self._default_scopes = default_scopes
+
+        if self._client_id:
+            self._client_auth = utils.ClientAuthentication(
+                utils.ClientAuthType.basic, self._client_id, self._client_secret
+            )
+        else:
+            self._client_auth = None
+        self._sts_client = sts.Client(self._token_url, self._client_auth)
+
+        if self._service_account_impersonation_url:
+            self._impersonated_credentials = self._initialize_impersonated_credentials()
+        else:
+            self._impersonated_credentials = None
+        self._project_id = None
+
+    @property
+    def requires_scopes(self):
+        """Checks if the credentials requires scopes.
+
+        Returns:
+            bool: True if there are no scopes set otherwise False.
+        """
+        return not self._scopes and not self._default_scopes
+
+    @property
+    def project_number(self):
+        """Optional[str]: The project number corresponding to the workload identity pool."""
+
+        # STS audience pattern:
+        # //iam.googleapis.com/projects/$PROJECT_NUMBER/locations/...
+        components = self._audience.split("/")
+        try:
+            project_index = components.index("projects")
+            if project_index + 1 < len(components):
+                return components[project_index + 1] or None
+        except ValueError:
+            return None
+
+    @_helpers.copy_docstring(credentials.Scoped)
+    def with_scopes(self, scopes, default_scopes=None):
+        return self.__class__(
+            audience=self._audience,
+            subject_token_type=self._subject_token_type,
+            token_url=self._token_url,
+            credential_source=self._credential_source,
+            service_account_impersonation_url=self._service_account_impersonation_url,
+            client_id=self._client_id,
+            client_secret=self._client_secret,
+            quota_project_id=self._quota_project_id,
+            scopes=scopes,
+            default_scopes=default_scopes,
+        )
+
+    @abc.abstractmethod
+    def retrieve_subject_token(self, request):
+        """Retrieves the subject token using the credential_source object.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+        Returns:
+            str: The retrieved subject token.
+        """
+        # pylint: disable=missing-raises-doc
+        # (pylint doesn't recognize that this is abstract)
+        raise NotImplementedError("retrieve_subject_token must be implemented")
+
+    def get_project_id(self, request):
+        """Retrieves the project ID corresponding to the workload identity pool.
+
+        When not determinable, None is returned.
+
+        This is introduced to support the current pattern of using the Auth library:
+
+            credentials, project_id = google.auth.default()
+
+        The resource may not have permission (resourcemanager.projects.get) to
+        call this API or the required scopes may not be selected:
+        https://cloud.google.com/resource-manager/reference/rest/v1/projects/get#authorization-scopes
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+        Returns:
+            Optional[str]: The project ID corresponding to the workload identity pool
+                if determinable.
+        """
+        if self._project_id:
+            # If already retrieved, return the cached project ID value.
+            return self._project_id
+        scopes = self._scopes if self._scopes is not None else self._default_scopes
+        # Scopes are required in order to retrieve a valid access token.
+        if self.project_number and scopes:
+            headers = {}
+            url = _CLOUD_RESOURCE_MANAGER + self.project_number
+            self.before_request(request, "GET", url, headers)
+            response = request(url=url, method="GET", headers=headers)
+
+            response_body = (
+                response.data.decode("utf-8")
+                if hasattr(response.data, "decode")
+                else response.data
+            )
+            response_data = json.loads(response_body)
+
+            if response.status == 200:
+                # Cache result as this field is immutable.
+                self._project_id = response_data.get("projectId")
+                return self._project_id
+
+        return None
+
+    @_helpers.copy_docstring(credentials.Credentials)
+    def refresh(self, request):
+        scopes = self._scopes if self._scopes is not None else self._default_scopes
+        if self._impersonated_credentials:
+            self._impersonated_credentials.refresh(request)
+            self.token = self._impersonated_credentials.token
+            self.expiry = self._impersonated_credentials.expiry
+        else:
+            now = _helpers.utcnow()
+            response_data = self._sts_client.exchange_token(
+                request=request,
+                grant_type=_STS_GRANT_TYPE,
+                subject_token=self.retrieve_subject_token(request),
+                subject_token_type=self._subject_token_type,
+                audience=self._audience,
+                scopes=scopes,
+                requested_token_type=_STS_REQUESTED_TOKEN_TYPE,
+            )
+            self.token = response_data.get("access_token")
+            lifetime = datetime.timedelta(seconds=response_data.get("expires_in"))
+            self.expiry = now + lifetime
+
+    @_helpers.copy_docstring(credentials.CredentialsWithQuotaProject)
+    def with_quota_project(self, quota_project_id):
+        # Return copy of instance with the provided quota project ID.
+        return self.__class__(
+            audience=self._audience,
+            subject_token_type=self._subject_token_type,
+            token_url=self._token_url,
+            credential_source=self._credential_source,
+            service_account_impersonation_url=self._service_account_impersonation_url,
+            client_id=self._client_id,
+            client_secret=self._client_secret,
+            quota_project_id=quota_project_id,
+            scopes=self._scopes,
+            default_scopes=self._default_scopes,
+        )
+
+    def _initialize_impersonated_credentials(self):
+        """Generates an impersonated credentials.
+
+        For more details, see `projects.serviceAccounts.generateAccessToken`_.
+
+        .. _projects.serviceAccounts.generateAccessToken: https://cloud.google.com/iam/docs/reference/credentials/rest/v1/projects.serviceAccounts/generateAccessToken
+
+        Returns:
+            impersonated_credentials.Credential: The impersonated credentials
+                object.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If the generateAccessToken
+                endpoint returned an error.
+        """
+        # Return copy of instance with no service account impersonation.
+        source_credentials = self.__class__(
+            audience=self._audience,
+            subject_token_type=self._subject_token_type,
+            token_url=self._token_url,
+            credential_source=self._credential_source,
+            service_account_impersonation_url=None,
+            client_id=self._client_id,
+            client_secret=self._client_secret,
+            quota_project_id=self._quota_project_id,
+            scopes=self._scopes,
+            default_scopes=self._default_scopes,
+        )
+
+        # Determine target_principal.
+        start_index = self._service_account_impersonation_url.rfind("/")
+        end_index = self._service_account_impersonation_url.find(":generateAccessToken")
+        if start_index != -1 and end_index != -1 and start_index < end_index:
+            start_index = start_index + 1
+            target_principal = self._service_account_impersonation_url[
+                start_index:end_index
+            ]
+        else:
+            raise exceptions.RefreshError(
+                "Unable to determine target principal from service account impersonation URL."
+            )
+
+        scopes = self._scopes if self._scopes is not None else self._default_scopes
+        # Initialize and return impersonated credentials.
+        return impersonated_credentials.Credentials(
+            source_credentials=source_credentials,
+            target_principal=target_principal,
+            target_scopes=scopes,
+            quota_project_id=self._quota_project_id,
+            iam_endpoint_override=self._service_account_impersonation_url,
+        )
diff --git a/google/auth/identity_pool.py b/google/auth/identity_pool.py
new file mode 100644
index 0000000..5362199
--- /dev/null
+++ b/google/auth/identity_pool.py
@@ -0,0 +1,279 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Identity Pool Credentials.
+
+This module provides credentials to access Google Cloud resources from on-prem
+or non-Google Cloud platforms which support external credentials (e.g. OIDC ID
+tokens) retrieved from local file locations or local servers. This includes
+Microsoft Azure and OIDC identity providers (e.g. K8s workloads registered with
+Hub with Hub workload identity enabled).
+
+These credentials are recommended over the use of service account credentials
+in on-prem/non-Google Cloud platforms as they do not involve the management of
+long-live service account private keys.
+
+Identity Pool Credentials are initialized using external_account
+arguments which are typically loaded from an external credentials file or
+an external credentials URL. Unlike other Credentials that can be initialized
+with a list of explicit arguments, secrets or credentials, external account
+clients use the environment and hints/guidelines provided by the
+external_account JSON file to retrieve credentials and exchange them for Google
+access tokens.
+"""
+
+try:
+    from collections.abc import Mapping
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from collections import Mapping
+import io
+import json
+import os
+
+from google.auth import _helpers
+from google.auth import exceptions
+from google.auth import external_account
+
+
+class Credentials(external_account.Credentials):
+    """External account credentials sourced from files and URLs."""
+
+    def __init__(
+        self,
+        audience,
+        subject_token_type,
+        token_url,
+        credential_source,
+        service_account_impersonation_url=None,
+        client_id=None,
+        client_secret=None,
+        quota_project_id=None,
+        scopes=None,
+        default_scopes=None,
+    ):
+        """Instantiates an external account credentials object from a file/URL.
+
+        Args:
+            audience (str): The STS audience field.
+            subject_token_type (str): The subject token type.
+            token_url (str): The STS endpoint URL.
+            credential_source (Mapping): The credential source dictionary used to
+                provide instructions on how to retrieve external credential to be
+                exchanged for Google access tokens.
+
+                Example credential_source for url-sourced credential::
+
+                    {
+                        "url": "http://www.example.com",
+                        "format": {
+                            "type": "json",
+                            "subject_token_field_name": "access_token",
+                        },
+                        "headers": {"foo": "bar"},
+                    }
+
+                Example credential_source for file-sourced credential::
+
+                    {
+                        "file": "/path/to/token/file.txt"
+                    }
+
+            service_account_impersonation_url (Optional[str]): The optional service account
+                impersonation getAccessToken URL.
+            client_id (Optional[str]): The optional client ID.
+            client_secret (Optional[str]): The optional client secret.
+            quota_project_id (Optional[str]): The optional quota project ID.
+            scopes (Optional[Sequence[str]]): Optional scopes to request during the
+                authorization grant.
+            default_scopes (Optional[Sequence[str]]): Default scopes passed by a
+                Google client library. Use 'scopes' for user-defined scopes.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error is encountered during
+                access token retrieval logic.
+            ValueError: For invalid parameters.
+
+        .. note:: Typically one of the helper constructors
+            :meth:`from_file` or
+            :meth:`from_info` are used instead of calling the constructor directly.
+        """
+
+        super(Credentials, self).__init__(
+            audience=audience,
+            subject_token_type=subject_token_type,
+            token_url=token_url,
+            credential_source=credential_source,
+            service_account_impersonation_url=service_account_impersonation_url,
+            client_id=client_id,
+            client_secret=client_secret,
+            quota_project_id=quota_project_id,
+            scopes=scopes,
+            default_scopes=default_scopes,
+        )
+        if not isinstance(credential_source, Mapping):
+            self._credential_source_file = None
+            self._credential_source_url = None
+        else:
+            self._credential_source_file = credential_source.get("file")
+            self._credential_source_url = credential_source.get("url")
+            self._credential_source_headers = credential_source.get("headers")
+            credential_source_format = credential_source.get("format", {})
+            # Get credential_source format type. When not provided, this
+            # defaults to text.
+            self._credential_source_format_type = (
+                credential_source_format.get("type") or "text"
+            )
+            # environment_id is only supported in AWS or dedicated future external
+            # account credentials.
+            if "environment_id" in credential_source:
+                raise ValueError(
+                    "Invalid Identity Pool credential_source field 'environment_id'"
+                )
+            if self._credential_source_format_type not in ["text", "json"]:
+                raise ValueError(
+                    "Invalid credential_source format '{}'".format(
+                        self._credential_source_format_type
+                    )
+                )
+            # For JSON types, get the required subject_token field name.
+            if self._credential_source_format_type == "json":
+                self._credential_source_field_name = credential_source_format.get(
+                    "subject_token_field_name"
+                )
+                if self._credential_source_field_name is None:
+                    raise ValueError(
+                        "Missing subject_token_field_name for JSON credential_source format"
+                    )
+            else:
+                self._credential_source_field_name = None
+
+        if self._credential_source_file and self._credential_source_url:
+            raise ValueError(
+                "Ambiguous credential_source. 'file' is mutually exclusive with 'url'."
+            )
+        if not self._credential_source_file and not self._credential_source_url:
+            raise ValueError(
+                "Missing credential_source. A 'file' or 'url' must be provided."
+            )
+
+    @_helpers.copy_docstring(external_account.Credentials)
+    def retrieve_subject_token(self, request):
+        return self._parse_token_data(
+            self._get_token_data(request),
+            self._credential_source_format_type,
+            self._credential_source_field_name,
+        )
+
+    def _get_token_data(self, request):
+        if self._credential_source_file:
+            return self._get_file_data(self._credential_source_file)
+        else:
+            return self._get_url_data(
+                request, self._credential_source_url, self._credential_source_headers
+            )
+
+    def _get_file_data(self, filename):
+        if not os.path.exists(filename):
+            raise exceptions.RefreshError("File '{}' was not found.".format(filename))
+
+        with io.open(filename, "r", encoding="utf-8") as file_obj:
+            return file_obj.read(), filename
+
+    def _get_url_data(self, request, url, headers):
+        response = request(url=url, method="GET", headers=headers)
+
+        # support both string and bytes type response.data
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
+
+        if response.status != 200:
+            raise exceptions.RefreshError(
+                "Unable to retrieve Identity Pool subject token", response_body
+            )
+
+        return response_body, url
+
+    def _parse_token_data(
+        self, token_content, format_type="text", subject_token_field_name=None
+    ):
+        content, filename = token_content
+        if format_type == "text":
+            token = content
+        else:
+            try:
+                # Parse file content as JSON.
+                response_data = json.loads(content)
+                # Get the subject_token.
+                token = response_data[subject_token_field_name]
+            except (KeyError, ValueError):
+                raise exceptions.RefreshError(
+                    "Unable to parse subject_token from JSON file '{}' using key '{}'".format(
+                        filename, subject_token_field_name
+                    )
+                )
+        if not token:
+            raise exceptions.RefreshError(
+                "Missing subject_token in the credential_source file"
+            )
+        return token
+
+    @classmethod
+    def from_info(cls, info, **kwargs):
+        """Creates an Identity Pool Credentials instance from parsed external account info.
+
+        Args:
+            info (Mapping[str, str]): The Identity Pool external account info in Google
+                format.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.identity_pool.Credentials: The constructed
+                credentials.
+
+        Raises:
+            ValueError: For invalid parameters.
+        """
+        return cls(
+            audience=info.get("audience"),
+            subject_token_type=info.get("subject_token_type"),
+            token_url=info.get("token_url"),
+            service_account_impersonation_url=info.get(
+                "service_account_impersonation_url"
+            ),
+            client_id=info.get("client_id"),
+            client_secret=info.get("client_secret"),
+            credential_source=info.get("credential_source"),
+            quota_project_id=info.get("quota_project_id"),
+            **kwargs
+        )
+
+    @classmethod
+    def from_file(cls, filename, **kwargs):
+        """Creates an IdentityPool Credentials instance from an external account json file.
+
+        Args:
+            filename (str): The path to the IdentityPool external account json file.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.identity_pool.Credentials: The constructed
+                credentials.
+        """
+        with io.open(filename, "r", encoding="utf-8") as json_file:
+            data = json.load(json_file)
+            return cls.from_info(data, **kwargs)
diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index 4d15837..b8a6c49 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -65,7 +65,9 @@
 _DEFAULT_TOKEN_URI = "https://oauth2.googleapis.com/token"
 
 
-def _make_iam_token_request(request, principal, headers, body):
+def _make_iam_token_request(
+    request, principal, headers, body, iam_endpoint_override=None
+):
     """Makes a request to the Google Cloud IAM service for an access token.
     Args:
         request (Request): The Request object to use.
@@ -73,6 +75,9 @@
         headers (Mapping[str, str]): Map of headers to transmit.
         body (Mapping[str, str]): JSON Payload body for the iamcredentials
             API call.
+        iam_endpoint_override (Optiona[str]): The full IAM endpoint override
+            with the target_principal embedded. This is useful when supporting
+            impersonation with regional endpoints.
 
     Raises:
         google.auth.exceptions.TransportError: Raised if there is an underlying
@@ -82,7 +87,7 @@
             `iamcredentials.googleapis.com` is not enabled or the
             `Service Account Token Creator` is not assigned
     """
-    iam_endpoint = _IAM_ENDPOINT.format(principal)
+    iam_endpoint = iam_endpoint_override or _IAM_ENDPOINT.format(principal)
 
     body = json.dumps(body).encode("utf-8")
 
@@ -185,6 +190,7 @@
         delegates=None,
         lifetime=_DEFAULT_TOKEN_LIFETIME_SECS,
         quota_project_id=None,
+        iam_endpoint_override=None,
     ):
         """
         Args:
@@ -209,6 +215,9 @@
             quota_project_id (Optional[str]): The project ID used for quota and billing.
                 This project may be different from the project used to
                 create the credentials.
+            iam_endpoint_override (Optiona[str]): The full IAM endpoint override
+                with the target_principal embedded. This is useful when supporting
+                impersonation with regional endpoints.
         """
 
         super(Credentials, self).__init__()
@@ -226,6 +235,7 @@
         self.token = None
         self.expiry = _helpers.utcnow()
         self._quota_project_id = quota_project_id
+        self._iam_endpoint_override = iam_endpoint_override
 
     @_helpers.copy_docstring(credentials.Credentials)
     def refresh(self, request):
@@ -260,6 +270,7 @@
             principal=self._target_principal,
             headers=headers,
             body=body,
+            iam_endpoint_override=self._iam_endpoint_override,
         )
 
     def sign_bytes(self, message):
@@ -302,6 +313,7 @@
             delegates=self._delegates,
             lifetime=self._lifetime,
             quota_project_id=quota_project_id,
+            iam_endpoint_override=self._iam_endpoint_override,
         )
 
 
diff --git a/google/oauth2/sts.py b/google/oauth2/sts.py
new file mode 100644
index 0000000..ae3c014
--- /dev/null
+++ b/google/oauth2/sts.py
@@ -0,0 +1,155 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""OAuth 2.0 Token Exchange Spec.
+
+This module defines a token exchange utility based on the `OAuth 2.0 Token
+Exchange`_ spec. This will be mainly used to exchange external credentials
+for GCP access tokens in workload identity pools to access Google APIs.
+
+The implementation will support various types of client authentication as
+allowed in the spec.
+
+A deviation on the spec will be for additional Google specific options that
+cannot be easily mapped to parameters defined in the RFC.
+
+The returned dictionary response will be based on the `rfc8693 section 2.2.1`_
+spec JSON response.
+
+.. _OAuth 2.0 Token Exchange: https://tools.ietf.org/html/rfc8693
+.. _rfc8693 section 2.2.1: https://tools.ietf.org/html/rfc8693#section-2.2.1
+"""
+
+import json
+
+from six.moves import http_client
+from six.moves import urllib
+
+from google.oauth2 import utils
+
+
+_URLENCODED_HEADERS = {"Content-Type": "application/x-www-form-urlencoded"}
+
+
+class Client(utils.OAuthClientAuthHandler):
+    """Implements the OAuth 2.0 token exchange spec based on
+    https://tools.ietf.org/html/rfc8693.
+    """
+
+    def __init__(self, token_exchange_endpoint, client_authentication=None):
+        """Initializes an STS client instance.
+
+        Args:
+            token_exchange_endpoint (str): The token exchange endpoint.
+            client_authentication (Optional(google.oauth2.oauth2_utils.ClientAuthentication)):
+                The optional OAuth client authentication credentials if available.
+        """
+        super(Client, self).__init__(client_authentication)
+        self._token_exchange_endpoint = token_exchange_endpoint
+
+    def exchange_token(
+        self,
+        request,
+        grant_type,
+        subject_token,
+        subject_token_type,
+        resource=None,
+        audience=None,
+        scopes=None,
+        requested_token_type=None,
+        actor_token=None,
+        actor_token_type=None,
+        additional_options=None,
+        additional_headers=None,
+    ):
+        """Exchanges the provided token for another type of token based on the
+        rfc8693 spec.
+
+        Args:
+            request (google.auth.transport.Request): A callable used to make
+                HTTP requests.
+            grant_type (str): The OAuth 2.0 token exchange grant type.
+            subject_token (str): The OAuth 2.0 token exchange subject token.
+            subject_token_type (str): The OAuth 2.0 token exchange subject token type.
+            resource (Optional[str]): The optional OAuth 2.0 token exchange resource field.
+            audience (Optional[str]): The optional OAuth 2.0 token exchange audience field.
+            scopes (Optional[Sequence[str]]): The optional list of scopes to use.
+            requested_token_type (Optional[str]): The optional OAuth 2.0 token exchange requested
+                token type.
+            actor_token (Optional[str]): The optional OAuth 2.0 token exchange actor token.
+            actor_token_type (Optional[str]): The optional OAuth 2.0 token exchange actor token type.
+            additional_options (Optional[Mapping[str, str]]): The optional additional
+                non-standard Google specific options.
+            additional_headers (Optional[Mapping[str, str]]): The optional additional
+                headers to pass to the token exchange endpoint.
+
+        Returns:
+            Mapping[str, str]: The token exchange JSON-decoded response data containing
+                the requested token and its expiration time.
+
+        Raises:
+            google.auth.exceptions.OAuthError: If the token endpoint returned
+                an error.
+        """
+        # Initialize request headers.
+        headers = _URLENCODED_HEADERS.copy()
+        # Inject additional headers.
+        if additional_headers:
+            for k, v in dict(additional_headers).items():
+                headers[k] = v
+        # Initialize request body.
+        request_body = {
+            "grant_type": grant_type,
+            "resource": resource,
+            "audience": audience,
+            "scope": " ".join(scopes or []),
+            "requested_token_type": requested_token_type,
+            "subject_token": subject_token,
+            "subject_token_type": subject_token_type,
+            "actor_token": actor_token,
+            "actor_token_type": actor_token_type,
+            "options": None,
+        }
+        # Add additional non-standard options.
+        if additional_options:
+            request_body["options"] = urllib.parse.quote(json.dumps(additional_options))
+        # Remove empty fields in request body.
+        for k, v in dict(request_body).items():
+            if v is None or v == "":
+                del request_body[k]
+        # Apply OAuth client authentication.
+        self.apply_client_authentication_options(headers, request_body)
+
+        # Execute request.
+        response = request(
+            url=self._token_exchange_endpoint,
+            method="POST",
+            headers=headers,
+            body=urllib.parse.urlencode(request_body).encode("utf-8"),
+        )
+
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
+
+        # If non-200 response received, translate to OAuthError exception.
+        if response.status != http_client.OK:
+            utils.handle_error_response(response_body)
+
+        response_data = json.loads(response_body)
+
+        # Return successful response.
+        return response_data
diff --git a/google/oauth2/utils.py b/google/oauth2/utils.py
new file mode 100644
index 0000000..efda796
--- /dev/null
+++ b/google/oauth2/utils.py
@@ -0,0 +1,171 @@
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""OAuth 2.0 Utilities.
+
+This module provides implementations for various OAuth 2.0 utilities.
+This includes `OAuth error handling`_ and
+`Client authentication for OAuth flows`_.
+
+OAuth error handling
+--------------------
+This will define interfaces for handling OAuth related error responses as
+stated in `RFC 6749 section 5.2`_.
+This will include a common function to convert these HTTP error responses to a
+:class:`google.auth.exceptions.OAuthError` exception.
+
+
+Client authentication for OAuth flows
+-------------------------------------
+We introduce an interface for defining client authentication credentials based
+on `RFC 6749 section 2.3.1`_. This will expose the following
+capabilities:
+
+    * Ability to support basic authentication via request header.
+    * Ability to support bearer token authentication via request header.
+    * Ability to support client ID / secret authentication via request body.
+
+.. _RFC 6749 section 2.3.1: https://tools.ietf.org/html/rfc6749#section-2.3.1
+.. _RFC 6749 section 5.2: https://tools.ietf.org/html/rfc6749#section-5.2
+"""
+
+import abc
+import base64
+import enum
+import json
+
+import six
+
+from google.auth import exceptions
+
+
+# OAuth client authentication based on
+# https://tools.ietf.org/html/rfc6749#section-2.3.
+class ClientAuthType(enum.Enum):
+    basic = 1
+    request_body = 2
+
+
+class ClientAuthentication(object):
+    """Defines the client authentication credentials for basic and request-body
+    types based on https://tools.ietf.org/html/rfc6749#section-2.3.1.
+    """
+
+    def __init__(self, client_auth_type, client_id, client_secret=None):
+        """Instantiates a client authentication object containing the client ID
+        and secret credentials for basic and response-body auth.
+
+        Args:
+            client_auth_type (google.oauth2.oauth_utils.ClientAuthType): The
+                client authentication type.
+            client_id (str): The client ID.
+            client_secret (Optional[str]): The client secret.
+        """
+        self.client_auth_type = client_auth_type
+        self.client_id = client_id
+        self.client_secret = client_secret
+
+
+@six.add_metaclass(abc.ABCMeta)
+class OAuthClientAuthHandler(object):
+    """Abstract class for handling client authentication in OAuth-based
+    operations.
+    """
+
+    def __init__(self, client_authentication=None):
+        """Instantiates an OAuth client authentication handler.
+
+        Args:
+            client_authentication (Optional[google.oauth2.utils.ClientAuthentication]):
+                The OAuth client authentication credentials if available.
+        """
+        super(OAuthClientAuthHandler, self).__init__()
+        self._client_authentication = client_authentication
+
+    def apply_client_authentication_options(
+        self, headers, request_body=None, bearer_token=None
+    ):
+        """Applies client authentication on the OAuth request's headers or POST
+        body.
+
+        Args:
+            headers (Mapping[str, str]): The HTTP request header.
+            request_body (Optional[Mapping[str, str]): The HTTP request body
+                dictionary. For requests that do not support request body, this
+                is None and will be ignored.
+            bearer_token (Optional[str]): The optional bearer token.
+        """
+        # Inject authenticated header.
+        self._inject_authenticated_headers(headers, bearer_token)
+        # Inject authenticated request body.
+        if bearer_token is None:
+            self._inject_authenticated_request_body(request_body)
+
+    def _inject_authenticated_headers(self, headers, bearer_token=None):
+        if bearer_token is not None:
+            headers["Authorization"] = "Bearer %s" % bearer_token
+        elif (
+            self._client_authentication is not None
+            and self._client_authentication.client_auth_type is ClientAuthType.basic
+        ):
+            username = self._client_authentication.client_id
+            password = self._client_authentication.client_secret or ""
+
+            credentials = base64.b64encode(
+                ("%s:%s" % (username, password)).encode()
+            ).decode()
+            headers["Authorization"] = "Basic %s" % credentials
+
+    def _inject_authenticated_request_body(self, request_body):
+        if (
+            self._client_authentication is not None
+            and self._client_authentication.client_auth_type
+            is ClientAuthType.request_body
+        ):
+            if request_body is None:
+                raise exceptions.OAuthError(
+                    "HTTP request does not support request-body"
+                )
+            else:
+                request_body["client_id"] = self._client_authentication.client_id
+                request_body["client_secret"] = (
+                    self._client_authentication.client_secret or ""
+                )
+
+
+def handle_error_response(response_body):
+    """Translates an error response from an OAuth operation into an
+    OAuthError exception.
+
+    Args:
+        response_body (str): The decoded response data.
+
+    Raises:
+        google.auth.exceptions.OAuthError
+    """
+    try:
+        error_components = []
+        error_data = json.loads(response_body)
+
+        error_components.append("Error code {}".format(error_data["error"]))
+        if "error_description" in error_data:
+            error_components.append(": {}".format(error_data["error_description"]))
+        if "error_uri" in error_data:
+            error_components.append(" - {}".format(error_data["error_uri"]))
+        error_details = "".join(error_components)
+    # If no details could be extracted, use the response data.
+    except (KeyError, ValueError):
+        error_details = response_body
+
+    raise exceptions.OAuthError(error_details, response_body)