diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
new file mode 100644
index 0000000..dedde9e
--- /dev/null
+++ b/google/auth/exceptions.py
@@ -0,0 +1,32 @@
+# 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.
+
+"""Exceptions used in the google.auth package."""
+
+
+class GoogleAuthError(Exception):
+    """Base class for all google.auth errors."""
+
+
+class TransportError(Exception):
+    """Used to indicate an error occurred during an HTTP request."""
+
+
+class RefreshError(GoogleAuthError):
+    """Used to indicate that an refreshing the credentials' access token
+    failed."""
+
+
+class DefaultCredentialsError(GoogleAuthError):
+    """Used to indicate that acquiring default credentials failed."""
diff --git a/google/auth/transport/__init__.py b/google/auth/transport/__init__.py
new file mode 100644
index 0000000..32de8f1
--- /dev/null
+++ b/google/auth/transport/__init__.py
@@ -0,0 +1,85 @@
+# 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.
+
+"""Transport - HTTP client library support.
+
+:mod:`google.auth` is designed to work with various HTTP client libraries such
+as urllib3 and requests. In order to work across these libraries with different
+interfaces some abstraction is needed.
+
+This module provides two interfaces that are implemented by transport adapters
+to support HTTP libraries. :class:`Request` defines the interface expected by
+:mod:`google.auth` to make requests. :class:`Response` defines the interface
+for the return value of :class:`Request`.
+"""
+
+import abc
+
+import six
+
+
+@six.add_metaclass(abc.ABCMeta)
+class Response(object):
+    """HTTP Response data."""
+
+    @abc.abstractproperty
+    def status(self):
+        """int: The HTTP status code."""
+        raise NotImplementedError('status must be implemented.')
+
+    @abc.abstractproperty
+    def headers(self):
+        """Mapping: The HTTP response headers."""
+        raise NotImplementedError('headers must be implemented.')
+
+    @abc.abstractproperty
+    def data(self):
+        """bytes: The response body."""
+        raise NotImplementedError('data must be implemented.')
+
+
+@six.add_metaclass(abc.ABCMeta)
+class Request(object):
+    """Interface for a callable that makes HTTP requests.
+
+    Specific transport implementations should provide an implementation of
+    this that adapts their specific request / response API.
+    """
+
+    @abc.abstractmethod
+    def __call__(self, url, method='GET', body=None, headers=None,
+                 timeout=None, **kwargs):
+        """Make an HTTP request.
+
+        Args:
+            url (str): The URI to be requested.
+            method (str): The HTTP method to use for the request. Defaults
+                to 'GET'.
+            body (bytes): The payload / body in HTTP request.
+            headers (Mapping): Request headers.
+            timeout (Optional(int)): The number of seconds to wait for a
+                response from the server. If not specified or if None, the
+                transport-specific default timeout will be used.
+            kwargs: Additionally arguments passed on to the transport's
+                request method.
+
+        Returns:
+            Response: The HTTP response.
+
+        Raises:
+            google.auth.exceptions.TransportError: If any exception occurred.
+        """
+        # pylint: disable=redundant-returns-doc, missing-raises-doc
+        # (pylint doesn't play well with abstract docstrings.)
+        raise NotImplementedError('__call__ must be implemented.')