diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index f68fc4b..394a7d5 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -42,8 +42,13 @@
 
 import base64
 import collections
+import datetime
+import io
 import json
 
+from six.moves import urllib
+
+from google.auth import credentials
 from google.auth import crypt
 from google.auth import _helpers
 
@@ -234,3 +239,240 @@
                     claim_audience, audience))
 
     return payload
+
+
+class Credentials(credentials.Signing,
+                  credentials.Credentials):
+    """Credentials that use a JWT as the bearer token.
+
+    These credentials require an "audience" claim. This claim identifies the
+    intended recipient of the bearer token. You can set the audience when
+    you construct these credentials, however, these credentials can also set
+    the audience claim automatically if not specified. In this case, whenever
+    a request is made the credentials will automatically generate a one-time
+    JWT with the request URI as the audience.
+
+    The constructor arguments determine the claims for the JWT that is
+    sent with requests. Usually, you'll construct these credentials with
+    one of the helper constructors as shown in the next section.
+
+    To create JWT credentials using a Google service account private key
+    JSON file::
+
+        credentials = jwt.Credentials.from_service_account_file(
+            'service-account.json')
+
+    If you already have the service account file loaded and parsed::
+
+        service_account_info = json.load(open('service_account.json'))
+        credentials = jwt.Credentials.from_service_account_info(
+            service_account_info)
+
+    Both helper methods pass on arguments to the constructor, so you can
+    specify the JWT claims::
+
+        credentials = jwt.Credentials.from_service_account_file(
+            'service-account.json',
+            audience='https://speech.googleapis.com',
+            additional_claims={'meta': 'data'})
+
+    You can also construct the credentials directly if you have a
+    :class:`~google.auth.crypt.Signer` instance::
+
+        credentials = jwt.Credentials(
+            signer, issuer='your-issuer', subject='your-subject')
+
+    The claims are considered immutable. If you want to modify the claims,
+    you can easily create another instance using :meth:`with_claims`::
+
+        new_credentials = credentials.with_claims(
+            audience='https://vision.googleapis.com')
+    """
+
+    def __init__(self, signer, issuer=None, subject=None, audience=None,
+                 additional_claims=None,
+                 token_lifetime=_DEFAULT_TOKEN_LIFETIME_SECS):
+        """
+        Args:
+            signer (google.auth.crypt.Signer): The signer used to sign JWTs.
+            issuer (str): The `iss` claim.
+            subject (str): The `sub` claim.
+            audience (str): the `aud` claim. The intended audience for the
+                credentials. If not specified, a new JWT will be generated for
+                every request and will use the request URI as the audience.
+            additional_claims (Mapping[str, str]): Any additional claims for
+                the JWT payload.
+            token_lifetime (int): The amount of time in seconds for
+                which the token is valid. Defaults to 1 hour.
+        """
+        super(Credentials, self).__init__()
+        self._signer = signer
+        self._issuer = issuer
+        self._subject = subject
+        self._audience = audience
+        self._additional_claims = additional_claims or {}
+        self._token_lifetime = token_lifetime
+
+    @classmethod
+    def from_service_account_info(cls, info, **kwargs):
+        """Creates a Credentials instance from parsed service account info.
+
+        Args:
+            info (Mapping[str, str]): The service account info in Google
+                format.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.Credentials: The constructed credentials.
+
+        Raises:
+            ValueError: If the info is not in the expected format.
+        """
+
+        try:
+            email = info['client_email']
+            key_id = info['private_key_id']
+            private_key = info['private_key']
+        except KeyError:
+            raise ValueError(
+                'Service account info was not in the expected format.')
+
+        signer = crypt.Signer.from_string(private_key, key_id)
+
+        kwargs.setdefault('subject', email)
+        return cls(signer, issuer=email, **kwargs)
+
+    @classmethod
+    def from_service_account_file(cls, filename, **kwargs):
+        """Creates a Credentials instance from a service account json file.
+
+        Args:
+            filename (str): The path to the service account json file.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.jwt.Credentials: The constructed credentials.
+        """
+        with io.open(filename, 'r', encoding='utf-8') as json_file:
+            info = json.load(json_file)
+        return cls.from_service_account_info(info, **kwargs)
+
+    def with_claims(self, issuer=None, subject=None, audience=None,
+                    additional_claims=None):
+        """Returns a copy of these credentials with modified claims.
+
+        Args:
+            issuer (str): The `iss` claim. If unspecified the current issuer
+                claim will be used.
+            subject (str): The `sub` claim. If unspecified the current subject
+                claim will be used.
+            audience (str): the `aud` claim. If not specified, a new
+                JWT will be generated for every request and will use
+                the request URI as the audience.
+            additional_claims (Mapping[str, str]): Any additional claims for
+                the JWT payload. This will be merged with the current
+                additional claims.
+
+        Returns:
+            google.auth.jwt.Credentials: A new credentials instance.
+        """
+        return Credentials(
+            self._signer,
+            issuer=issuer if issuer is not None else self._issuer,
+            subject=subject if subject is not None else self._subject,
+            audience=audience if audience is not None else self._audience,
+            additional_claims=self._additional_claims.copy().update(
+                additional_claims or {}))
+
+    def _make_jwt(self, audience=None):
+        """Make a signed JWT.
+
+        Args:
+            audience (str): Overrides the instance's current audience claim.
+
+        Returns:
+            Tuple(bytes, datetime): The encoded JWT and the expiration.
+        """
+        now = _helpers.utcnow()
+        lifetime = datetime.timedelta(seconds=self._token_lifetime)
+        expiry = now + lifetime
+
+        payload = {
+            'iss': self._issuer,
+            'sub': self._subject or self._issuer,
+            'iat': _helpers.datetime_to_secs(now),
+            'exp': _helpers.datetime_to_secs(expiry),
+            'aud': audience or self._audience,
+        }
+
+        payload.update(self._additional_claims)
+
+        jwt = encode(self._signer, payload)
+
+        return jwt, expiry
+
+    def _make_one_time_jwt(self, uri):
+        """Makes a one-off JWT with the URI as the audience.
+
+        Args:
+            uri (str): The request URI.
+
+        Returns:
+            bytes: The encoded JWT.
+        """
+        parts = urllib.parse.urlsplit(uri)
+        # Strip query string and fragment
+        audience = urllib.parse.urlunsplit(
+            (parts.scheme, parts.netloc, parts.path, None, None))
+        token, _ = self._make_jwt(audience=audience)
+        return token
+
+    def refresh(self, request):
+        """Refreshes the access token.
+
+        Args:
+            request (Any): Unused.
+        """
+        # pylint: disable=unused-argument
+        # (pylint doesn't correctly recognize overridden methods.)
+        self.token, self.expiry = self._make_jwt()
+
+    def sign_bytes(self, message):
+        """Signs the given message.
+
+        Args:
+            message (bytes): The message to sign.
+
+        Returns:
+            bytes: The message signature.
+        """
+        return self._signer.sign(message)
+
+    def before_request(self, request, method, url, headers):
+        """Performs credential-specific before request logic.
+
+        If an audience is specified it will refresh the credentials if
+        necessary. If no audience is specified it will generate a one-time
+        token for the request URI. In either case, it will set the
+        authorization header in headers to the token.
+
+        Args:
+            request (Any): Unused.
+            method (str): The request's HTTP method.
+            url (str): The request's URI.
+            headers (Mapping): The request's headers.
+        """
+        # pylint: disable=unused-argument
+        # (pylint doesn't correctly recognize overridden methods.)
+
+        # If this set of credentials has a pre-set audience, just ensure that
+        # there is a valid token and apply the auth headers.
+        if self._audience:
+            if not self.valid:
+                self.refresh(request)
+            self.apply(headers)
+        # Otherwise, generate a one-time token using the URL
+        # (without the query string and fragment) as the audience.
+        else:
+            token = self._make_one_time_jwt(url)
+            self.apply(headers, token=token)
diff --git a/tests/test_jwt.py b/tests/test_jwt.py
index 69628e5..b6c07df 100644
--- a/tests/test_jwt.py
+++ b/tests/test_jwt.py
@@ -14,8 +14,10 @@
 
 import base64
 import datetime
+import json
 import os
 
+import mock
 import pytest
 
 from google.auth import _helpers
@@ -34,6 +36,11 @@
 with open(os.path.join(DATA_DIR, 'other_cert.pem'), 'rb') as fh:
     OTHER_CERT_BYTES = fh.read()
 
+SERVICE_ACCOUNT_JSON_FILE = os.path.join(DATA_DIR, 'service_account.json')
+
+with open(SERVICE_ACCOUNT_JSON_FILE, 'r') as fh:
+    SERVICE_ACCOUNT_INFO = json.load(fh)
+
 
 @pytest.fixture
 def signer():
@@ -187,3 +194,147 @@
     certs = {'2': OTHER_CERT_BYTES, '3': PUBLIC_CERT_BYTES}
     payload = jwt.decode(token, certs)
     assert payload['user'] == 'billy bob'
+
+
+class TestCredentials:
+    SERVICE_ACCOUNT_EMAIL = 'service-account@example.com'
+    SUBJECT = 'subject'
+    AUDIENCE = 'audience'
+    ADDITIONAL_CLAIMS = {'meta': 'data'}
+    credentials = None
+
+    @pytest.fixture(autouse=True)
+    def credentials_fixture(self, signer):
+        self.credentials = jwt.Credentials(
+            signer, self.SERVICE_ACCOUNT_EMAIL)
+
+    def test_from_service_account_info(self):
+        with open(SERVICE_ACCOUNT_JSON_FILE, 'r') as fh:
+            info = json.load(fh)
+
+        credentials = jwt.Credentials.from_service_account_info(info)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == info['client_email']
+
+    def test_from_service_account_info_args(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.Credentials.from_service_account_info(
+            info, subject=self.SUBJECT, audience=self.AUDIENCE,
+            additional_claims=self.ADDITIONAL_CLAIMS)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == self.SUBJECT
+        assert credentials._audience == self.AUDIENCE
+        assert credentials._additional_claims == self.ADDITIONAL_CLAIMS
+
+    def test_from_service_account_bad_private_key(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+        info['private_key'] = 'garbage'
+
+        with pytest.raises(ValueError) as excinfo:
+            jwt.Credentials.from_service_account_info(info)
+
+        assert excinfo.match(r'No key could be detected')
+
+    def test_from_service_account_bad_format(self):
+        with pytest.raises(ValueError):
+            jwt.Credentials.from_service_account_info({})
+
+    def test_from_service_account_file(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.Credentials.from_service_account_file(
+            SERVICE_ACCOUNT_JSON_FILE)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == info['client_email']
+
+    def test_from_service_account_file_args(self):
+        info = SERVICE_ACCOUNT_INFO.copy()
+
+        credentials = jwt.Credentials.from_service_account_file(
+            SERVICE_ACCOUNT_JSON_FILE, subject=self.SUBJECT,
+            audience=self.AUDIENCE, additional_claims=self.ADDITIONAL_CLAIMS)
+
+        assert credentials._signer.key_id == info['private_key_id']
+        assert credentials._issuer == info['client_email']
+        assert credentials._subject == self.SUBJECT
+        assert credentials._audience == self.AUDIENCE
+        assert credentials._additional_claims == self.ADDITIONAL_CLAIMS
+
+    def test_default_state(self):
+        assert not self.credentials.valid
+        # Expiration hasn't been set yet
+        assert not self.credentials.expired
+
+    def test_sign_bytes(self):
+        to_sign = b'123'
+        signature = self.credentials.sign_bytes(to_sign)
+        assert crypt.verify_signature(to_sign, signature, PUBLIC_CERT_BYTES)
+
+    def _verify_token(self, token):
+        payload = jwt.decode(token, PUBLIC_CERT_BYTES)
+        assert payload['iss'] == self.SERVICE_ACCOUNT_EMAIL
+        return payload
+
+    def test_refresh(self):
+        self.credentials.refresh(None)
+        assert self.credentials.valid
+        assert not self.credentials.expired
+
+    def test_expired(self):
+        assert not self.credentials.expired
+
+        self.credentials.refresh(None)
+        assert not self.credentials.expired
+
+        with mock.patch('google.auth._helpers.utcnow') as now:
+            one_day = datetime.timedelta(days=1)
+            now.return_value = self.credentials.expiry + one_day
+            assert self.credentials.expired
+
+    def test_before_request_one_time_token(self):
+        headers = {}
+
+        self.credentials.refresh(None)
+        self.credentials.before_request(
+            mock.Mock(), 'GET', 'http://example.com?a=1#3', headers)
+
+        header_value = headers['authorization']
+        _, token = header_value.split(' ')
+
+        # This should be a one-off token, so it shouldn't be the same as the
+        # credentials' stored token.
+        assert token != self.credentials.token
+
+        payload = self._verify_token(token)
+        assert payload['aud'] == 'http://example.com'
+
+    def test_before_request_with_preset_audience(self):
+        headers = {}
+
+        credentials = self.credentials.with_claims(audience=self.AUDIENCE)
+        credentials.refresh(None)
+        credentials.before_request(
+            None, 'GET', 'http://example.com?a=1#3', headers)
+
+        header_value = headers['authorization']
+        _, token = header_value.split(' ')
+
+        # Since the audience is set, it should use the existing token.
+        assert token.encode('utf-8') == credentials.token
+
+        payload = self._verify_token(token)
+        assert payload['aud'] == self.AUDIENCE
+
+    def test_before_request_refreshes(self):
+        credentials = self.credentials.with_claims(audience=self.AUDIENCE)
+        assert not credentials.valid
+        credentials.before_request(
+            None, 'GET', 'http://example.com?a=1#3', {})
+        assert credentials.valid