diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
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+# Copyright 2018 Google Inc.
+#
+# 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.
+
+"""Google Cloud Impersonated credentials.
+
+This module provides authentication for applications where local credentials
+impersonates a remote service account using `IAM Credentials API`_.
+
+This class can be used to impersonate a service account as long as the original
+Credential object has the "Service Account Token Creator" role on the target
+service account.
+
+    .. _IAM Credentials API:
+        https://cloud.google.com/iam/credentials/reference/rest/
+"""
+
+import copy
+from datetime import datetime
+import json
+
+import six
+from six.moves import http_client
+
+from google.auth import _helpers
+from google.auth import credentials
+from google.auth import exceptions
+
+_DEFAULT_TOKEN_LIFETIME_SECS = 3600  # 1 hour in seconds
+
+_IAM_SCOPE = ['https://www.googleapis.com/auth/iam']
+
+_IAM_ENDPOINT = ('https://iamcredentials.googleapis.com/v1/projects/-' +
+                 '/serviceAccounts/{}:generateAccessToken')
+
+_REFRESH_ERROR = 'Unable to acquire impersonated credentials'
+_LIFETIME_ERROR = 'Credentials with lifetime set cannot be renewed'
+
+
+def _make_iam_token_request(request, principal, headers, body):
+    """Makes a request to the Google Cloud IAM service for an access token.
+    Args:
+        request (Request): The Request object to use.
+        principal (str): The principal to request an access token for.
+        headers (Mapping[str, str]): Map of headers to transmit.
+        body (Mapping[str, str]): JSON Payload body for the iamcredentials
+            API call.
+
+    Raises:
+        TransportError: Raised if there is an underlying HTTP connection
+        Error
+        DefaultCredentialsError: Raised if the impersonated credentials
+        are not available.  Common reasons are
+        `iamcredentials.googleapis.com` is not enabled or the
+        `Service Account Token Creator` is not assigned
+    """
+    iam_endpoint = _IAM_ENDPOINT.format(principal)
+
+    body = json.dumps(body)
+
+    response = request(
+        url=iam_endpoint,
+        method='POST',
+        headers=headers,
+        body=body)
+
+    response_body = response.data.decode('utf-8')
+
+    if response.status != http_client.OK:
+        exceptions.RefreshError(_REFRESH_ERROR, response_body)
+
+    try:
+        token_response = json.loads(response.data.decode('utf-8'))
+        token = token_response['accessToken']
+        expiry = datetime.strptime(
+            token_response['expireTime'], '%Y-%m-%dT%H:%M:%SZ')
+
+        return token, expiry
+
+    except (KeyError, ValueError) as caught_exc:
+        new_exc = exceptions.RefreshError(
+            '{}: No access token or invalid expiration in response.'.format(
+                _REFRESH_ERROR),
+            response_body)
+        six.raise_from(new_exc, caught_exc)
+
+
+class Credentials(credentials.Credentials):
+    """This module defines impersonated credentials which are essentially
+    impersonated identities.
+
+    Impersonated Credentials allows credentials issued to a user or
+    service account to impersonate another. The target service account must
+    grant the originating credential principal the
+    `Service Account Token Creator`_ IAM role:
+
+    For more information about Token Creator IAM role and
+    IAMCredentials API, see
+    `Creating Short-Lived Service Account Credentials`_.
+
+    .. _Service Account Token Creator:
+        https://cloud.google.com/iam/docs/service-accounts#the_service_account_token_creator_role
+
+    .. _Creating Short-Lived Service Account Credentials:
+        https://cloud.google.com/iam/docs/creating-short-lived-service-account-credentials
+
+    Usage:
+
+    First grant source_credentials the `Service Account Token Creator`
+    role on the target account to impersonate.   In this example, the
+    service account represented by svc_account.json has the
+    token creator role on
+    `impersonated-account@_project_.iam.gserviceaccount.com`.
+
+    Initialize a source credential which does not have access to
+    list bucket::
+
+        from google.oauth2 import service_acccount
+
+        target_scopes = [
+            'https://www.googleapis.com/auth/devstorage.read_only']
+
+        source_credentials = (
+            service_account.Credentials.from_service_account_file(
+                '/path/to/svc_account.json',
+                scopes=target_scopes))
+
+    Now use the source credentials to acquire credentials to impersonate
+    another service account::
+
+        from google.auth import impersonated_credentials
+
+        target_credentials = impersonated_credentials.Credentials(
+          source_credentials=source_credentials,
+          target_principal='impersonated-account@_project_.iam.gserviceaccount.com',
+          target_scopes = target_scopes,
+          lifetime=500)
+
+    Resource access is granted::
+
+        client = storage.Client(credentials=target_credentials)
+        buckets = client.list_buckets(project='your_project')
+        for bucket in buckets:
+          print bucket.name
+    """
+
+    def __init__(self, source_credentials,  target_principal,
+                 target_scopes, delegates=None,
+                 lifetime=None):
+        """
+        Args:
+            source_credentials (google.auth.Credentials): The source credential
+                used as to acquire the impersonated credentials.
+            target_principal (str): The service account to impersonate.
+            target_scopes (Sequence[str]): Scopes to request during the
+                authorization grant.
+            delegates (Sequence[str]): The chained list of delegates required
+                to grant the final access_token.  If set, the sequence of
+                identities must have "Service Account Token Creator" capability
+                granted to the prceeding identity.  For example, if set to
+                [serviceAccountB, serviceAccountC], the source_credential
+                must have the Token Creator role on serviceAccountB.
+                serviceAccountB must have the Token Creator on serviceAccountC.
+                Finally, C must have Token Creator on target_principal.
+                If left unset, source_credential must have that role on
+                target_principal.
+            lifetime (int): Number of seconds the delegated credential should
+                be valid for (upto 3600).  If set, the credentials will
+                **not** get refreshed after expiration.  If not set, the
+                credentials will be refreshed every 3600s.
+        """
+
+        super(Credentials, self).__init__()
+
+        self._source_credentials = copy.copy(source_credentials)
+        self._source_credentials._scopes = _IAM_SCOPE
+        self._target_principal = target_principal
+        self._target_scopes = target_scopes
+        self._delegates = delegates
+        self._lifetime = lifetime
+        self.token = None
+        self.expiry = _helpers.utcnow()
+
+    @_helpers.copy_docstring(credentials.Credentials)
+    def refresh(self, request):
+        if (self.token is not None and self._lifetime is not None):
+            self.expiry = _helpers.utcnow()
+            raise exceptions.RefreshError(_LIFETIME_ERROR)
+        self._source_credentials.refresh(request)
+        self._update_token(request)
+
+    @property
+    def expired(self):
+        return _helpers.utcnow() >= self.expiry
+
+    def _update_token(self, request):
+        """Updates credentials with a new access_token representing
+        the impersonated account.
+
+        Args:
+            request (google.auth.transport.requests.Request): Request object
+                to use for refreshing credentials.
+        """
+
+        # Refresh our source credentials.
+        self._source_credentials.refresh(request)
+
+        lifetime = self._lifetime
+        if (self._lifetime is None):
+            lifetime = _DEFAULT_TOKEN_LIFETIME_SECS
+
+        body = {
+            "delegates": self._delegates,
+            "scope": self._target_scopes,
+            "lifetime": str(lifetime) + "s"
+        }
+
+        headers = {
+            'Content-Type': 'application/json',
+        }
+
+        # Apply the source credentials authentication info.
+        self._source_credentials.apply(headers)
+
+        self.token, self.expiry = _make_iam_token_request(
+            request=request,
+            principal=self._target_principal,
+            headers=headers,
+            body=body)