diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
new file mode 100644
index 0000000..1b26549
--- /dev/null
+++ b/google/oauth2/_client.py
@@ -0,0 +1,200 @@
+# 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
diff --git a/tests/oauth2/__init__.py b/tests/oauth2/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/tests/oauth2/__init__.py
diff --git a/tests/oauth2/test__client.py b/tests/oauth2/test__client.py
new file mode 100644
index 0000000..8c19c3e
--- /dev/null
+++ b/tests/oauth2/test__client.py
@@ -0,0 +1,169 @@
+# 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.
+
+import datetime
+import json
+
+import mock
+import pytest
+import six
+from six.moves import http_client
+from six.moves import urllib
+
+from google.auth import exceptions
+from google.oauth2 import _client
+
+
+def test__handle_error_response():
+    response_data = json.dumps({
+        'error': 'help',
+        'error_description': 'I\'m alive'})
+
+    with pytest.raises(exceptions.RefreshError) as excinfo:
+        _client._handle_error_response(response_data)
+
+    assert excinfo.match(r'help: I\'m alive')
+
+
+def test__handle_error_response_non_json():
+    response_data = 'Help, I\'m alive'
+
+    with pytest.raises(exceptions.RefreshError) as excinfo:
+        _client._handle_error_response(response_data)
+
+    assert excinfo.match(r'Help, I\'m alive')
+
+
+@mock.patch('google.auth._helpers.utcnow', return_value=datetime.datetime.min)
+def test__parse_expiry(now_mock):
+    result = _client._parse_expiry({'expires_in': 500})
+    assert result == datetime.datetime.min + datetime.timedelta(seconds=500)
+
+
+def test__parse_expiry_none():
+    assert _client._parse_expiry({}) is None
+
+
+def _make_request(response_data):
+    response = mock.Mock()
+    response.status = http_client.OK
+    response.data = json.dumps(response_data).encode('utf-8')
+    return mock.Mock(return_value=response)
+
+
+def test__token_endpoint_request():
+    request = _make_request({'test': 'response'})
+
+    result = _client._token_endpoint_request(
+        request, 'http://example.com', {'test': 'params'})
+
+    # Check request call
+    request.assert_called_with(
+        method='POST',
+        url='http://example.com',
+        headers={'content-type': 'application/x-www-form-urlencoded'},
+        body='test=params')
+
+    # Check result
+    assert result == {'test': 'response'}
+
+
+def test__token_endpoint_request_error():
+    response = mock.Mock()
+    response.status = http_client.BAD_REQUEST
+    response.data = b'Error'
+    request = mock.Mock(return_value=response)
+
+    with pytest.raises(exceptions.RefreshError):
+        _client._token_endpoint_request(request, 'http://example.com', {})
+
+
+def _verify_request_params(request, params):
+    request_body = request.call_args[1]['body']
+    request_params = urllib.parse.parse_qs(request_body)
+
+    for key, value in six.iteritems(params):
+        assert request_params[key][0] == value
+
+
+@mock.patch('google.auth._helpers.utcnow', return_value=datetime.datetime.min)
+def test_jwt_grant(now_mock):
+    request = _make_request({
+        'access_token': 'token',
+        'expires_in': 500,
+        'extra': 'data'})
+
+    token, expiry, extra_data = _client.jwt_grant(
+        request, 'http://example.com', 'assertion_value')
+
+    # Check request call
+    _verify_request_params(request, {
+        'grant_type': _client._JWT_GRANT_TYPE,
+        'assertion': 'assertion_value'
+    })
+
+    # Check result
+    assert token == 'token'
+    assert expiry == datetime.datetime.min + datetime.timedelta(seconds=500)
+    assert extra_data['extra'] == 'data'
+
+
+def test_jwt_grant_no_access_token():
+    request = _make_request({
+        # No access token.
+        'expires_in': 500,
+        'extra': 'data'})
+
+    with pytest.raises(exceptions.RefreshError):
+        _client.jwt_grant(request, 'http://example.com', 'assertion_value')
+
+
+@mock.patch('google.auth._helpers.utcnow', return_value=datetime.datetime.min)
+def test_refresh_grant(now_mock):
+    request = _make_request({
+        'access_token': 'token',
+        'refresh_token': 'new_refresh_token',
+        'expires_in': 500,
+        'extra': 'data'})
+
+    token, refresh_token, expiry, extra_data = _client.refresh_grant(
+        request, 'http://example.com', 'refresh_token', 'client_id',
+        'client_secret')
+
+    # Check request call
+    _verify_request_params(request, {
+        'grant_type': _client._REFRESH_GRANT_TYPE,
+        'refresh_token': 'refresh_token',
+        'client_id': 'client_id',
+        'client_secret': 'client_secret'
+    })
+
+    # Check result
+    assert token == 'token'
+    assert refresh_token == 'new_refresh_token'
+    assert expiry == datetime.datetime.min + datetime.timedelta(seconds=500)
+    assert extra_data['extra'] == 'data'
+
+
+def test_refresh_grant_no_access_token():
+    request = _make_request({
+        # No access token.
+        'refresh_token': 'new_refresh_token',
+        'expires_in': 500,
+        'extra': 'data'})
+
+    with pytest.raises(exceptions.RefreshError):
+        _client.refresh_grant(
+            request, 'http://example.com', 'refresh_token', 'client_id',
+            'client_secret')