diff --git a/lib/googleauth.rb b/lib/googleauth.rb
new file mode 100644
index 0000000..d6dafa5
--- /dev/null
+++ b/lib/googleauth.rb
@@ -0,0 +1,66 @@
+# 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_ERROR = <<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.
+    #
+    # Use this to obtain the Application Default Credentials for accessing
+    # Google APIs.  Application Default Credentials are described in detail
+    # at http://goo.gl/IUuyuX.
+    #
+    # 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_ERROR unless GCECredentials.on_gce?(options)
+      GCECredentials.new
+    end
+
+    module_function :get_application_default
+  end
+end
diff --git a/lib/googleauth/compute_engine.rb b/lib/googleauth/compute_engine.rb
index c99214b..b820af1 100644
--- a/lib/googleauth/compute_engine.rb
+++ b/lib/googleauth/compute_engine.rb
@@ -46,6 +46,7 @@ module Google
 
       class << self
         extend Memoist
+
         # Detect if this appear to be a GCE instance, by checking if metadata
         # is available
         def on_gce?(options = {})
@@ -64,9 +65,10 @@ module Google
           return false unless resp.status == 200
           return false unless resp.headers.key?('Metadata-Flavor')
           return resp.headers['Metadata-Flavor'] == 'Google'
-        rescue [Faraday::TimeoutError, Faraday::ConnectionFailed]
+        rescue Faraday::TimeoutError, Faraday::ConnectionFailed
           return false
         end
+
         memoize :on_gce?
       end
 
diff --git a/lib/googleauth/service_account.rb b/lib/googleauth/service_account.rb
index a7b9072..4e8072b 100644
--- a/lib/googleauth/service_account.rb
+++ b/lib/googleauth/service_account.rb
@@ -28,8 +28,10 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 require 'googleauth/signet'
+require 'memoist'
 require 'multi_json'
 require 'openssl'
+require 'rbconfig'
 
 # Reads the private key and client email fields from service account JSON key.
 def read_json_key(json_key_io)
@@ -45,14 +47,59 @@ module Google
   module Auth
     # Authenticates requests using Google's Service Account credentials.
     #
-    # This class provides a simpler surface to the behavior in
-    # Signet::OAuth2::Client.  It allows authorizing requests directly using
-    # credentials from a json key file downloaded from the developer console
-    # (via 'Generate new Json Key').
+    # 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').
     #
     # cf [Application Default Credentials](http://goo.gl/mkAHpZ)
     class ServiceAccountCredentials < Signet::OAuth2::Client
+      ENV_VAR = 'GOOGLE_APPLICATION_CREDENTIALS'
+      NOT_FOUND_ERROR =
+        "Unable to read the credential file specified by #{ENV_VAR}"
       TOKEN_CRED_URI = 'https://www.googleapis.com/oauth2/v3/token'
+      WELL_KNOWN_PATH = 'gcloud/application_default_credentials.json'
+      WELL_KNOWN_ERROR = 'Unable to read the default credential file'
+
+      class << self
+        extend Memoist
+
+        # determines if the current OS is windows
+        def windows?
+          RbConfig::CONFIG['host_os'] =~ /Windows|mswin/
+        end
+        memoize :windows?
+
+        # Creates an instance from the path specified in an environment
+        # variable.
+        #
+        # @param scope [string|array] the scope(s) to access
+        def from_env(scope)
+          return nil unless ENV.key?(ENV_VAR)
+          path = ENV[ENV_VAR]
+          fail 'file #{path} does not exist' unless File.exist?(path)
+          File.open(path) do |f|
+            return new(scope, f)
+          end
+        rescue StandardError => e
+          raise "#{NOT_FOUND_ERROR}: #{e}"
+        end
+
+        # Creates an instance from a well known path.
+        #
+        # @param scope [string|array] the scope(s) to access
+        def from_well_known_path(scope)
+          home_var, base = windows? ? 'APPDATA' : 'HOME', WELL_KNOWN_PATH
+          root = ENV[home_var].nil? ? '' : ENV[home_var]
+          base = File.join('.config', base) unless windows?
+          path = File.join(root, base)
+          return nil unless File.exist?(path)
+          File.open(path) do |f|
+            return new(scope, f)
+          end
+        rescue StandardError => e
+          raise "#{WELL_KNOWN_ERROR}: #{e}"
+        end
+      end
 
       # Initializes a ServiceAccountCredentials.
       #
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
diff --git a/spec/googleauth/service_account_spec.rb b/spec/googleauth/service_account_spec.rb
index 20700d1..e6657b4 100644
--- a/spec/googleauth/service_account_spec.rb
+++ b/spec/googleauth/service_account_spec.rb
@@ -32,24 +32,20 @@ $LOAD_PATH.unshift(spec_dir)
 $LOAD_PATH.uniq!
 
 require 'apply_auth_examples'
+require 'fileutils'
 require 'googleauth/service_account'
 require 'jwt'
 require 'multi_json'
 require 'openssl'
 require 'spec_helper'
+require 'tmpdir'
 
 describe Google::Auth::ServiceAccountCredentials do
+  ServiceAccountCredentials = Google::Auth::ServiceAccountCredentials
+
   before(:example) do
     @key = OpenSSL::PKey::RSA.new(2048)
-    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'
-    }
-    cred_json_text = MultiJson.dump(cred_json)
-    @client = Google::Auth::ServiceAccountCredentials.new(
+    @client = ServiceAccountCredentials.new(
         'https://www.googleapis.com/auth/userinfo.profile',
         StringIO.new(cred_json_text))
   end
@@ -67,5 +63,81 @@ describe Google::Auth::ServiceAccountCredentials do
     end
   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_behaves_like 'apply/apply! are OK'
+
+  describe '#from_env' do
+    before(:example) do
+      @var_name = ServiceAccountCredentials::ENV_VAR
+      @orig = ENV[@var_name]
+      @scope = 'https://www.googleapis.com/auth/userinfo.profile'
+    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(ServiceAccountCredentials.from_env(@scope)).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(ServiceAccountCredentials.from_env(@scope)).to be_nil
+      Dir.mktmpdir do |dir|
+        key_path = File.join(dir, 'does-not-exist')
+        ENV[@var_name] = key_path
+        expect { sac.from_env(@scope) }.to raise_error
+      end
+    end
+
+    it 'succeeds when the GOOGLE_APPLICATION_CREDENTIALS file is valid' do
+      sac = ServiceAccountCredentials  # shortens name
+      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(sac.from_env(@scope)).to_not be_nil
+      end
+    end
+  end
+
+  describe '#from_well_known_path' do
+    before(:example) do
+      @home = ENV['HOME']
+      @scope = 'https://www.googleapis.com/auth/userinfo.profile'
+    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(ServiceAccountCredentials.from_well_known_path(@scope)).to be_nil
+    end
+
+    it 'successfully loads the file when it is present' do
+      sac = ServiceAccountCredentials  # shortens name
+      Dir.mktmpdir do |dir|
+        key_path = File.join(dir, '.config', sac::WELL_KNOWN_PATH)
+        FileUtils.mkdir_p(File.dirname(key_path))
+        File.write(key_path, cred_json_text)
+        ENV['HOME'] = dir
+        expect(sac.from_well_known_path(@scope)).to_not be_nil
+      end
+    end
+  end
 end