diff --git a/lib/googleauth.rb b/lib/googleauth.rb
new file mode 100644
index 0000000..4abc0f7
--- /dev/null
+++ b/lib/googleauth.rb
@@ -0,0 +1,62 @@
+# Copyright 2015, Google Inc.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#     * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+require 'googleauth/service_account'
+require 'googleauth/compute_engine'
+
+module Google
+  # Module Auth provides classes that provide Google-specific authorization
+  # used to access Google APIs.
+  module Auth
+    NOT_FOUND = <<END
+Could not load the default credentials. Browse to
+https://developers.google.com/accounts/docs/application-default-credentials
+for more information
+END
+
+    # Obtains the default credentials implementation to use in this
+    # environment.
+    #
+    # If supplied, scope is used to create the credentials instance, when it
+    # can applied.  E.g, on compute engine, the scope is ignored.
+    #
+    # @param scope [string|array] the scope(s) to access
+    # @param options [hash] allows override of the connection being used
+    def get_application_default(scope, options = {})
+      creds = ServiceAccountCredentials.from_env(scope)
+      return creds unless creds.nil?
+      creds = ServiceAccountCredentials.from_well_known_path(scope)
+      return creds unless creds.nil?
+      fail NOT_FOUND unless GCECredentials.on_gce?(options)
+      GCECredentials.new
+    end
+
+    module_function :get_application_default
+  end
+end
diff --git a/spec/googleauth/get_application_default_spec.rb b/spec/googleauth/get_application_default_spec.rb
new file mode 100644
index 0000000..6c1b9d5
--- /dev/null
+++ b/spec/googleauth/get_application_default_spec.rb
@@ -0,0 +1,134 @@
+# Copyright 2015, Google Inc.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#     * Neither the name of Google Inc. nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+spec_dir = File.expand_path(File.join(File.dirname(__FILE__)))
+$LOAD_PATH.unshift(spec_dir)
+$LOAD_PATH.uniq!
+
+require 'faraday'
+require 'googleauth'
+require 'spec_helper'
+
+describe '#get_application_default' do
+  before(:example) do
+    @key = OpenSSL::PKey::RSA.new(2048)
+    @var_name = ServiceAccountCredentials::ENV_VAR
+    @orig = ENV[@var_name]
+    @home = ENV['HOME']
+    @scope = 'https://www.googleapis.com/auth/userinfo.profile'
+  end
+
+  after(:example) do
+    ENV[@var_name] = @orig unless @orig.nil?
+    ENV['HOME'] = @home unless @home == ENV['HOME']
+  end
+
+  def cred_json_text
+    cred_json = {
+      private_key_id: 'a_private_key_id',
+      private_key: @key.to_pem,
+      client_email: 'app@developer.gserviceaccount.com',
+      client_id: 'app.apps.googleusercontent.com',
+      type: 'service_account'
+    }
+    MultiJson.dump(cred_json)
+  end
+
+  it 'fails if the GOOGLE_APPLICATION_CREDENTIALS path does not exist' do
+    Dir.mktmpdir do |dir|
+      key_path = File.join(dir, 'does-not-exist')
+      ENV[@var_name] = key_path
+      expect { Google::Auth.get_application_default(@scope) }.to raise_error
+    end
+  end
+
+  it 'succeeds if 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(Google::Auth.get_application_default(@scope)).to_not be_nil
+    end
+  end
+
+  it 'succeeds with default file if GOOGLE_APPLICATION_CREDENTIALS is unset' do
+    ENV.delete(@var_name) unless ENV[@var_name].nil?
+    Dir.mktmpdir do |dir|
+      key_path = File.join(dir, '.config',
+                           ServiceAccountCredentials::WELL_KNOWN_PATH)
+      FileUtils.mkdir_p(File.dirname(key_path))
+      File.write(key_path, cred_json_text)
+      ENV['HOME'] = dir
+      expect(Google::Auth.get_application_default(@scope)).to_not be_nil
+    end
+  end
+
+  it 'fails without default file or env if not on compute engine' do
+    stubs = Faraday::Adapter::Test::Stubs.new do |stub|
+      stub.get('/') do |_env|
+        [404,
+         { 'Metadata-Flavor' => 'Google' },
+         '']
+      end
+    end  # GCE not detected
+    Dir.mktmpdir do |dir|
+      ENV.delete(@var_name) unless ENV[@var_name].nil? # no env var
+      ENV['HOME'] = dir  # no config present in this tmp dir
+      c = Faraday.new do |b|
+        b.adapter(:test, stubs)
+      end
+      blk = proc do
+        Google::Auth.get_application_default(@scope, connection: c)
+      end
+      expect(&blk).to raise_error
+    end
+    stubs.verify_stubbed_calls
+  end
+
+  it 'succeeds without default file or env if on compute engine' do
+    stubs = Faraday::Adapter::Test::Stubs.new do |stub|
+      stub.get('/') do |_env|
+        [200,
+         { 'Metadata-Flavor' => 'Google' },
+         '']
+      end
+    end  # GCE detected
+    Dir.mktmpdir do |dir|
+      ENV.delete(@var_name) unless ENV[@var_name].nil? # no env var
+      ENV['HOME'] = dir  # no config present in this tmp dir
+      c = Faraday.new do |b|
+        b.adapter(:test, stubs)
+      end
+      expect(Google::Auth.get_application_default(@scope,
+                                                  connection: c)).to_not be_nil
+    end
+    stubs.verify_stubbed_calls
+  end
+end