diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
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+++ b/google/oauth2/_client.py
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+# Copyright 2016 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.
+
+"""OAuth 2.0 client.
+
+This is a client for interacting with an OAuth 2.0 authorization server's
+token endpoint.
+
+For more information about the token endpoint, see
+`Section 3.1 of rfc6749`_
+
+.. _Section 3.1 of rfc6749: https://tools.ietf.org/html/rfc6749#section-3.2
+"""
+
+import datetime
+import json
+
+from six.moves import http_client
+from six.moves import urllib
+
+from google.auth import _helpers
+from google.auth import exceptions
+
+_URLENCODED_CONTENT_TYPE = 'application/x-www-form-urlencoded'
+_JWT_GRANT_TYPE = 'urn:ietf:params:oauth:grant-type:jwt-bearer'
+_REFRESH_GRANT_TYPE = 'refresh_token'
+
+
+def _handle_error_response(response_body):
+    """"Translates an error response into an exception.
+
+    Args:
+        response_body (str): The decoded response data.
+
+    Raises:
+        google.auth.exceptions.RefreshError
+    """
+    try:
+        error_data = json.loads(response_body)
+        error_details = ': '.join([
+            error_data['error'],
+            error_data.get('error_description')])
+    # If no details could be extracted, use the response data.
+    except (KeyError, ValueError):
+        error_details = response_body
+
+    raise exceptions.RefreshError(
+        error_details, response_body)
+
+
+def _parse_expiry(response_data):
+    """Parses the expiry field from a response into a datetime.
+
+    Args:
+        response_data (Mapping): The JSON-parsed response data.
+
+    Returns:
+        Optional[datetime]: The expiration or ``None`` if no expiration was
+            specified.
+    """
+    expires_in = response_data.get('expires_in', None)
+
+    if expires_in is not None:
+        return _helpers.utcnow() + datetime.timedelta(
+            seconds=expires_in)
+    else:
+        return None
+
+
+def _token_endpoint_request(request, token_uri, body):
+    """Makes a request to the OAuth 2.0 authorization server's token endpoint.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        token_uri (str): The OAuth 2.0 authorizations server's token endpoint
+            URI.
+        body (Mapping[str, str]): The parameters to send in the request body.
+
+    Returns:
+        Mapping[str, str]: The JSON-decoded response data.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+    """
+    body = urllib.parse.urlencode(body)
+    headers = {
+        'content-type': _URLENCODED_CONTENT_TYPE,
+    }
+
+    response = request(
+        method='POST', url=token_uri, headers=headers, body=body)
+
+    response_body = response.data.decode('utf-8')
+
+    if response.status != http_client.OK:
+        _handle_error_response(response_body)
+
+    response_data = json.loads(response_body)
+
+    return response_data
+
+
+def jwt_grant(request, token_uri, assertion):
+    """Implements the JWT Profile for OAuth 2.0 Authorization Grants.
+
+    For more details, see `rfc7523 section 4`_.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        token_uri (str): The OAuth 2.0 authorizations server's token endpoint
+            URI.
+        assertion (str): The OAuth 2.0 assertion.
+
+    Returns:
+        Tuple[str, Optional[datetime], Mapping[str, str]]: The access token,
+            expiration, and additional data returned by the token endpoint.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+
+    .. _rfc7523 section 4: https://tools.ietf.org/html/rfc7523#section-4
+    """
+    body = {
+        'assertion': assertion,
+        'grant_type': _JWT_GRANT_TYPE,
+    }
+
+    response_data = _token_endpoint_request(request, token_uri, body)
+
+    try:
+        access_token = response_data['access_token']
+    except KeyError:
+        raise exceptions.RefreshError(
+            'No access token in response.', response_data)
+
+    expiry = _parse_expiry(response_data)
+
+    return access_token, expiry, response_data
+
+
+def refresh_grant(request, token_uri, refresh_token, client_id, client_secret):
+    """Implements the OAuth 2.0 refresh token grant.
+
+    For more details, see `rfc678 section 6`_.
+
+    Args:
+        request (google.auth.transport.Request): A callable used to make
+            HTTP requests.
+        token_uri (str): The OAuth 2.0 authorizations server's token endpoint
+            URI.
+        refresh_token (str): The refresh token to use to get a new access
+            token.
+        client_id (str): The OAuth 2.0 application's client ID.
+        client_secret (str): The Oauth 2.0 appliaction's client secret.
+
+    Returns:
+        Tuple[str, Optional[str], Optional[datetime], Mapping[str, str]]: The
+            access token, new refresh token, expiration, and additional data
+            returned by the token endpoint.
+
+    Raises:
+        google.auth.exceptions.RefreshError: If the token endpoint returned
+            an error.
+
+    .. _rfc6748 section 6: https://tools.ietf.org/html/rfc6749#section-6
+    """
+    body = {
+        'grant_type': _REFRESH_GRANT_TYPE,
+        'client_id': client_id,
+        'client_secret': client_secret,
+        'refresh_token': refresh_token,
+    }
+
+    response_data = _token_endpoint_request(request, token_uri, body)
+
+    try:
+        access_token = response_data['access_token']
+    except KeyError:
+        raise exceptions.RefreshError(
+            'No access token in response.', response_data)
+
+    refresh_token = response_data.get('refresh_token', refresh_token)
+    expiry = _parse_expiry(response_data)
+
+    return access_token, refresh_token, expiry, response_data