diff --git a/lib/googleauth/service_account.rb b/lib/googleauth/service_account.rb
index 076f5b1..8018e2e 100644
--- a/lib/googleauth/service_account.rb
+++ b/lib/googleauth/service_account.rb
@@ -36,7 +36,8 @@ 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
@@ -44,8 +45,6 @@ module Google
     #
     # cf [Application Default Credentials](http://goo.gl/mkAHpZ)
     class ServiceAccountCredentials < Signet::OAuth2::Client
-      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'
       extend CredentialsLoader
 
@@ -70,33 +69,94 @@ module Google
               issuer: client_email,
               signing_key: OpenSSL::PKey::RSA.new(private_key))
       end
+    end
 
-      # Extends the superclass behaviour to construct a jwt token if the
-      # google jwt uri key is present in the input hash.
+    # 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
       #
-      # The jwt is used as the authentication token.
+      # 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 WT_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)
-        unless jwt_aud_uri.nil?
-          jwt_token = new_jwt_token(jwt_aud_uri, opts)
-          a_hash[AUTH_METADATA_KEY] = "Bearer #{jwt_token}"
-          return a_hash
-        end
-        super
+        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,
+          'iss' => @issuer,
+          'sub' => @issuer,
           'aud' => jwt_aud_uri,
-          'exp' => (now + expiry).to_i,
+          'exp' => (now + EXPIRY).to_i,
           'iat' => (now - skew).to_i
         }
-        JWT.encode(assertion, signing_key, signing_algorithm)
+        JWT.encode(assertion, @signing_key, SIGNING_ALGORITHM)
       end
     end
   end
diff --git a/spec/googleauth/service_account_spec.rb b/spec/googleauth/service_account_spec.rb
index 6992b36..ee0f535 100644
--- a/spec/googleauth/service_account_spec.rb
+++ b/spec/googleauth/service_account_spec.rb
@@ -80,67 +80,6 @@ describe Google::Auth::ServiceAccountCredentials do
 
   it_behaves_like 'apply/apply! are OK'
 
-  context 'when jwt_aud_uri is present' do
-    WANTED_AUTH_KEY = ServiceAccountCredentials::AUTH_METADATA_KEY
-    JWT_AUD_URI_KEY = ServiceAccountCredentials::JWT_AUD_URI_KEY
-    let(:test_uri) { 'https://www.googleapis.com/myservice' }
-    let(:auth_prefix) { 'Bearer ' }
-
-    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_AUD_URI_KEY] = test_uri
-        @client.apply!(md)
-        auth_header = md[WANTED_AUTH_KEY]
-        expect_is_encoded_jwt(auth_header)
-        expect(md[JWT_AUD_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_AUD_URI_KEY] = test_uri
-        the_proc = @client.updater_proc
-        got = the_proc.call(md)
-        auth_header = got[WANTED_AUTH_KEY]
-        expect_is_encoded_jwt(auth_header)
-        expect(got[JWT_AUD_URI_KEY]).to be_nil
-        expect(md[JWT_AUD_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_AUD_URI_KEY] = test_uri
-        the_proc = @client.updater_proc
-        got = the_proc.call(md)
-        auth_header = md[WANTED_AUTH_KEY]
-        expect(auth_header).to be_nil
-        expect(got[JWT_AUD_URI_KEY]).to be_nil
-        expect(md[JWT_AUD_URI_KEY]).to_not be_nil
-      end
-
-      it 'should add a jwt token to the returned hash' do
-        md = { foo: 'bar' }
-        md[JWT_AUD_URI_KEY] = test_uri
-        got = @client.apply(md)
-        auth_header = got[WANTED_AUTH_KEY]
-        expect_is_encoded_jwt(auth_header)
-      end
-    end
-  end
-
   describe '#from_env' do
     before(:example) do
       @var_name = CredentialsLoader::ENV_VAR
@@ -207,3 +146,151 @@ describe Google::Auth::ServiceAccountCredentials do
     end
   end
 end
+
+describe Google::Auth::ServiceAccountJwtHeaderCredentials do
+  CredentialsLoader = Google::Auth::CredentialsLoader
+  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
+
+  context 'when jwt_aud_uri is present' do
+    WANTED_AUTH_KEY = ServiceAccountJwtHeaderCredentials::AUTH_METADATA_KEY
+    JWT_AUD_URI_KEY = ServiceAccountJwtHeaderCredentials::JWT_AUD_URI_KEY
+    let(:test_uri) { 'https://www.googleapis.com/myservice' }
+    let(:auth_prefix) { 'Bearer ' }
+
+    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_AUD_URI_KEY] = test_uri
+        @client.apply!(md)
+        auth_header = md[WANTED_AUTH_KEY]
+        expect_is_encoded_jwt(auth_header)
+        expect(md[JWT_AUD_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_AUD_URI_KEY] = test_uri
+        the_proc = @client.updater_proc
+        got = the_proc.call(md)
+        auth_header = got[WANTED_AUTH_KEY]
+        expect_is_encoded_jwt(auth_header)
+        expect(got[JWT_AUD_URI_KEY]).to be_nil
+        expect(md[JWT_AUD_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_AUD_URI_KEY] = test_uri
+        the_proc = @client.updater_proc
+        got = the_proc.call(md)
+        auth_header = md[WANTED_AUTH_KEY]
+        expect(auth_header).to be_nil
+        expect(got[JWT_AUD_URI_KEY]).to be_nil
+        expect(md[JWT_AUD_URI_KEY]).to_not be_nil
+      end
+
+      it 'should add a jwt token to the returned hash' do
+        md = { foo: 'bar' }
+        md[JWT_AUD_URI_KEY] = test_uri
+        got = @client.apply(md)
+        auth_header = got[WANTED_AUTH_KEY]
+        expect_is_encoded_jwt(auth_header)
+      end
+    end
+  end
+
+  describe '#from_env' do
+    before(:example) do
+      @var_name = CredentialsLoader::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']
+      @known_path = CredentialsLoader::WELL_KNOWN_PATH
+    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', @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