diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 543d13f..8c99036 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -1,15 +1,15 @@
 # This configuration was generated by `rubocop --auto-gen-config`
-# on 2015-02-25 04:34:33 -0800 using RuboCop version 0.28.0.
+# on 2015-03-06 19:51:00 -0800 using RuboCop version 0.28.0.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
 # versions of RuboCop, may require this file to be generated again.
 
-# Offense count: 1
+# Offense count: 3
 Metrics/AbcSize:
-  Max: 16
+  Max: 24
 
-# Offense count: 1
+# Offense count: 3
 # Configuration parameters: CountComments.
 Metrics/MethodLength:
   Max: 11
diff --git a/lib/googleauth/service_account.rb b/lib/googleauth/service_account.rb
index 66d7be6..c74d3cc 100644
--- a/lib/googleauth/service_account.rb
+++ b/lib/googleauth/service_account.rb
@@ -29,13 +29,16 @@
 
 require 'googleauth/signet'
 require 'googleauth/credentials_loader'
+require 'jwt'
 require 'multi_json'
+require 'stringio'
 
 module Google
   # Module Auth provides classes that provide Google-specific authorization
   # used to access Google APIs.
   module Auth
-    # Authenticates requests using Google's Service Account credentials.
+    # Authenticates requests using Google's Service Account credentials via an
+    # OAuth access token.
     #
     # This class allows authorizing requests for service accounts directly
     # from credentials from a json key file downloaded from the developer
@@ -67,6 +70,118 @@ module Google
               issuer: client_email,
               signing_key: OpenSSL::PKey::RSA.new(private_key))
       end
+
+      # Extends the base class.
+      #
+      # If scope(s) is not set, it creates a transient
+      # ServiceAccountJwtHeaderCredentials instance and uses that to
+      # authenticate instead.
+      def apply!(a_hash, opts = {})
+        # Use the base implementation if scopes are set
+        unless scope.nil?
+          super
+          return
+        end
+
+        # Use the ServiceAccountJwtHeaderCredentials using the same cred values
+        # if no scopes are set.
+        cred_json = {
+          private_key: @signing_key.to_s,
+          client_email: @issuer
+        }
+        alt_clz = ServiceAccountJwtHeaderCredentials
+        alt = alt_clz.new(StringIO.new(MultiJson.dump(cred_json)))
+        alt.apply!(a_hash)
+      end
+    end
+
+    # Authenticates requests using Google's Service Account credentials via
+    # JWT Header.
+    #
+    # This class allows authorizing requests for service accounts directly
+    # from credentials from a json key file downloaded from the developer
+    # console (via 'Generate new Json Key').  It is not part of any OAuth2
+    # flow, rather it creates a JWT and sends that as a credential.
+    #
+    # cf [Application Default Credentials](http://goo.gl/mkAHpZ)
+    class ServiceAccountJwtHeaderCredentials
+      JWT_AUD_URI_KEY = :jwt_aud_uri
+      AUTH_METADATA_KEY = Signet::OAuth2::AUTH_METADATA_KEY
+      TOKEN_CRED_URI = 'https://www.googleapis.com/oauth2/v3/token'
+      SIGNING_ALGORITHM = 'RS256'
+      EXPIRY = 60
+      extend CredentialsLoader
+
+      # make_creds proxies the construction of a credentials instance
+      #
+      # make_creds is used by the methods in CredentialsLoader.
+      #
+      # By default, it calls #new with 2 args, the second one being an
+      # optional scope. Here's the constructor only has one param, so
+      # we modify make_creds to reflect this.
+      def self.make_creds(*args)
+        new(args[0])
+      end
+
+      # Reads the private key and client email fields from the service account
+      # JSON key.
+      def self.read_json_key(json_key_io)
+        json_key = MultiJson.load(json_key_io.read)
+        fail 'missing client_email' unless json_key.key?('client_email')
+        fail 'missing private_key' unless json_key.key?('private_key')
+        [json_key['private_key'], json_key['client_email']]
+      end
+
+      # Initializes a ServiceAccountJwtHeaderCredentials.
+      #
+      # @param json_key_io [IO] an IO from which the JSON key can be read
+      def initialize(json_key_io)
+        private_key, client_email = self.class.read_json_key(json_key_io)
+        @private_key = private_key
+        @issuer = client_email
+        @signing_key = OpenSSL::PKey::RSA.new(private_key)
+      end
+
+      # Construct a jwt token if the JWT_AUD_URI key is present in the input
+      # hash.
+      #
+      # The jwt token is used as the value of a 'Bearer '.
+      def apply!(a_hash, opts = {})
+        jwt_aud_uri = a_hash.delete(JWT_AUD_URI_KEY)
+        return a_hash if jwt_aud_uri.nil?
+        jwt_token = new_jwt_token(jwt_aud_uri, opts)
+        a_hash[AUTH_METADATA_KEY] = "Bearer #{jwt_token}"
+        a_hash
+      end
+
+      # Returns a clone of a_hash updated with the authoriation header
+      def apply(a_hash, opts = {})
+        a_copy = a_hash.clone
+        apply!(a_copy, opts)
+        a_copy
+      end
+
+      # Returns a reference to the #apply method, suitable for passing as
+      # a closure
+      def updater_proc
+        lambda(&method(:apply))
+      end
+
+      protected
+
+      # Creates a jwt uri token.
+      def new_jwt_token(jwt_aud_uri, options = {})
+        now = Time.new
+        skew = options[:skew] || 60
+        assertion = {
+          'iss' => @issuer,
+          'sub' => @issuer,
+          'aud' => jwt_aud_uri,
+          'exp' => (now + EXPIRY).to_i,
+          'iat' => (now - skew).to_i
+        }
+        JWT.encode(assertion, @signing_key, SIGNING_ALGORITHM)
+      end
     end
   end
 end
diff --git a/spec/googleauth/apply_auth_examples.rb b/spec/googleauth/apply_auth_examples.rb
index fcafa9e..a0eca97 100644
--- a/spec/googleauth/apply_auth_examples.rb
+++ b/spec/googleauth/apply_auth_examples.rb
@@ -46,9 +46,9 @@ def build_access_token_json(token)
                       'expires_in' => 3600)
 end
 
-WANTED_AUTH_KEY = :Authorization
-
 shared_examples 'apply/apply! are OK' do
+  let(:auth_key) { :Authorization }
+
   # tests that use these examples need to define
   #
   # @client which should be an auth client
@@ -79,7 +79,7 @@ shared_examples 'apply/apply! are OK' do
 
       md = { foo: 'bar' }
       @client.apply!(md, connection: c)
-      want = { :foo => 'bar', WANTED_AUTH_KEY => "Bearer #{token}" }
+      want = { :foo => 'bar', auth_key => "Bearer #{token}" }
       expect(md).to eq(want)
       stubs.verify_stubbed_calls
     end
@@ -96,7 +96,7 @@ shared_examples 'apply/apply! are OK' do
       md = { foo: 'bar' }
       the_proc = @client.updater_proc
       got = the_proc.call(md, connection: c)
-      want = { :foo => 'bar', WANTED_AUTH_KEY => "Bearer #{token}" }
+      want = { :foo => 'bar', auth_key => "Bearer #{token}" }
       expect(got).to eq(want)
       stubs.verify_stubbed_calls
     end
@@ -126,7 +126,7 @@ shared_examples 'apply/apply! are OK' do
 
       md = { foo: 'bar' }
       got = @client.apply(md, connection: c)
-      want = { :foo => 'bar', WANTED_AUTH_KEY => "Bearer #{token}" }
+      want = { :foo => 'bar', auth_key => "Bearer #{token}" }
       expect(got).to eq(want)
       stubs.verify_stubbed_calls
     end
@@ -142,7 +142,7 @@ shared_examples 'apply/apply! are OK' do
       n.times do |_t|
         md = { foo: 'bar' }
         got = @client.apply(md, connection: c)
-        want = { :foo => 'bar', WANTED_AUTH_KEY => "Bearer #{token}" }
+        want = { :foo => 'bar', auth_key => "Bearer #{token}" }
         expect(got).to eq(want)
       end
       stubs.verify_stubbed_calls
@@ -159,7 +159,7 @@ shared_examples 'apply/apply! are OK' do
         end
         md = { foo: 'bar' }
         got = @client.apply(md, connection: c)
-        want = { :foo => 'bar', WANTED_AUTH_KEY => "Bearer #{t}" }
+        want = { :foo => 'bar', auth_key => "Bearer #{t}" }
         expect(got).to eq(want)
         stubs.verify_stubbed_calls
         @client.expires_at -= 3601 # default is to expire in 1hr
diff --git a/spec/googleauth/service_account_spec.rb b/spec/googleauth/service_account_spec.rb
index 108e2b0..7d5570a 100644
--- a/spec/googleauth/service_account_spec.rb
+++ b/spec/googleauth/service_account_spec.rb
@@ -40,9 +40,74 @@ require 'openssl'
 require 'spec_helper'
 require 'tmpdir'
 
+include Google::Auth::CredentialsLoader
+
+shared_examples 'jwt header auth' do
+  context 'when jwt_aud_uri is present' do
+    let(:test_uri) { 'https://www.googleapis.com/myservice' }
+    let(:auth_prefix) { 'Bearer ' }
+    let(:auth_key) { ServiceAccountJwtHeaderCredentials::AUTH_METADATA_KEY }
+    let(:jwt_uri_key) { ServiceAccountJwtHeaderCredentials::JWT_AUD_URI_KEY }
+
+    def expect_is_encoded_jwt(hdr)
+      expect(hdr).to_not be_nil
+      expect(hdr.start_with?(auth_prefix)).to be true
+      authorization = hdr[auth_prefix.length..-1]
+      payload, _ = JWT.decode(authorization, @key.public_key)
+      expect(payload['aud']).to eq(test_uri)
+      expect(payload['iss']).to eq(client_email)
+    end
+
+    describe '#apply!' do
+      it 'should update the target hash with a jwt token' do
+        md = { foo: 'bar' }
+        md[jwt_uri_key] = test_uri
+        @client.apply!(md)
+        auth_header = md[auth_key]
+        expect_is_encoded_jwt(auth_header)
+        expect(md[jwt_uri_key]).to be_nil
+      end
+    end
+
+    describe 'updater_proc' do
+      it 'should provide a proc that updates a hash with a jwt token' do
+        md = { foo: 'bar' }
+        md[jwt_uri_key] = test_uri
+        the_proc = @client.updater_proc
+        got = the_proc.call(md)
+        auth_header = got[auth_key]
+        expect_is_encoded_jwt(auth_header)
+        expect(got[jwt_uri_key]).to be_nil
+        expect(md[jwt_uri_key]).to_not be_nil
+      end
+    end
+
+    describe '#apply' do
+      it 'should not update the original hash with a jwt token' do
+        md = { foo: 'bar' }
+        md[jwt_uri_key] = test_uri
+        the_proc = @client.updater_proc
+        got = the_proc.call(md)
+        auth_header = md[auth_key]
+        expect(auth_header).to be_nil
+        expect(got[jwt_uri_key]).to be_nil
+        expect(md[jwt_uri_key]).to_not be_nil
+      end
+
+      it 'should add a jwt token to the returned hash' do
+        md = { foo: 'bar' }
+        md[jwt_uri_key] = test_uri
+        got = @client.apply(md)
+        auth_header = got[auth_key]
+        expect_is_encoded_jwt(auth_header)
+      end
+    end
+  end
+end
+
 describe Google::Auth::ServiceAccountCredentials do
   ServiceAccountCredentials = Google::Auth::ServiceAccountCredentials
-  CredentialsLoader = Google::Auth::CredentialsLoader
+  let(:client_email) { 'app@developer.gserviceaccount.com' }
 
   before(:example) do
     @key = OpenSSL::PKey::RSA.new(2048)
@@ -69,7 +134,7 @@ describe Google::Auth::ServiceAccountCredentials do
     cred_json = {
       private_key_id: 'a_private_key_id',
       private_key: @key.to_pem,
-      client_email: 'app@developer.gserviceaccount.com',
+      client_email: client_email,
       client_id: 'app.apps.googleusercontent.com',
       type: 'service_account'
     }
@@ -78,9 +143,17 @@ describe Google::Auth::ServiceAccountCredentials do
 
   it_behaves_like 'apply/apply! are OK'
 
+  context 'when scope is nil' do
+    before(:example) do
+      @client.scope = nil
+    end
+
+    it_behaves_like 'jwt header auth'
+  end
+
   describe '#from_env' do
     before(:example) do
-      @var_name = CredentialsLoader::ENV_VAR
+      @var_name = ENV_VAR
       @orig = ENV[@var_name]
       @scope = 'https://www.googleapis.com/auth/userinfo.profile'
       @clz = ServiceAccountCredentials
@@ -120,7 +193,7 @@ describe Google::Auth::ServiceAccountCredentials do
     before(:example) do
       @home = ENV['HOME']
       @scope = 'https://www.googleapis.com/auth/userinfo.profile'
-      @known_path = CredentialsLoader::WELL_KNOWN_PATH
+      @known_path = WELL_KNOWN_PATH
       @clz = ServiceAccountCredentials
     end
 
@@ -144,3 +217,90 @@ describe Google::Auth::ServiceAccountCredentials do
     end
   end
 end
+
+describe Google::Auth::ServiceAccountJwtHeaderCredentials do
+  ServiceAccountJwtHeaderCredentials =
+    Google::Auth::ServiceAccountJwtHeaderCredentials
+
+  let(:client_email) { 'app@developer.gserviceaccount.com' }
+  let(:clz) { Google::Auth::ServiceAccountJwtHeaderCredentials }
+
+  before(:example) do
+    @key = OpenSSL::PKey::RSA.new(2048)
+    @client = clz.new(StringIO.new(cred_json_text))
+  end
+
+  def cred_json_text
+    cred_json = {
+      private_key_id: 'a_private_key_id',
+      private_key: @key.to_pem,
+      client_email: client_email,
+      client_id: 'app.apps.googleusercontent.com',
+      type: 'service_account'
+    }
+    MultiJson.dump(cred_json)
+  end
+
+  it_behaves_like 'jwt header auth'
+
+  describe '#from_env' do
+    before(:example) do
+      @var_name = ENV_VAR
+      @orig = ENV[@var_name]
+    end
+
+    after(:example) do
+      ENV[@var_name] = @orig unless @orig.nil?
+    end
+
+    it 'returns nil if the GOOGLE_APPLICATION_CREDENTIALS is unset' do
+      ENV.delete(@var_name) unless ENV[@var_name].nil?
+      expect(clz.from_env).to be_nil
+    end
+
+    it 'fails if the GOOGLE_APPLICATION_CREDENTIALS path does not exist' do
+      ENV.delete(@var_name) unless ENV[@var_name].nil?
+      expect(clz.from_env).to be_nil
+      Dir.mktmpdir do |dir|
+        key_path = File.join(dir, 'does-not-exist')
+        ENV[@var_name] = key_path
+        expect { clz.from_env }.to raise_error
+      end
+    end
+
+    it 'succeeds when the GOOGLE_APPLICATION_CREDENTIALS file is valid' do
+      Dir.mktmpdir do |dir|
+        key_path = File.join(dir, 'my_cert_file')
+        FileUtils.mkdir_p(File.dirname(key_path))
+        File.write(key_path, cred_json_text)
+        ENV[@var_name] = key_path
+        expect(clz.from_env).to_not be_nil
+      end
+    end
+  end
+
+  describe '#from_well_known_path' do
+    before(:example) do
+      @home = ENV['HOME']
+    end
+
+    after(:example) do
+      ENV['HOME'] = @home unless @home == ENV['HOME']
+    end
+
+    it 'is nil if no file exists' do
+      ENV['HOME'] = File.dirname(__FILE__)
+      expect(clz.from_well_known_path).to be_nil
+    end
+
+    it 'successfully loads the file when it is present' do
+      Dir.mktmpdir do |dir|
+        key_path = File.join(dir, '.config', WELL_KNOWN_PATH)
+        FileUtils.mkdir_p(File.dirname(key_path))
+        File.write(key_path, cred_json_text)
+        ENV['HOME'] = dir
+        expect(clz.from_well_known_path).to_not be_nil
+      end
+    end
+  end
+end