diff --git a/railties/lib/rails/application.rb b/railties/lib/rails/application.rb
index b5c5a6191fd5c..6fd01ee768738 100644
--- a/railties/lib/rails/application.rb
+++ b/railties/lib/rails/application.rb
@@ -51,6 +51,29 @@ module Rails
   #   10) Run config.before_eager_load and eager_load! if eager_load is true
   #   11) Run config.after_initialize callbacks
   #
+  # == Multiple Applications
+  #
+  # If you decide to define multiple applications, then the first application
+  # that is initialized will be set to +Rails.application+, unless you override
+  # it with a different application.
+  #
+  # To create a new application, you can instantiate a new instance of a class
+  # that has already been created:
+  #
+  #   class Application < Rails::Application
+  #   end
+  #
+  #   first_application  = Application.new
+  #   second_application = Application.new(config: first_application.config)
+  #
+  # In the above example, the configuration from the first application was used
+  # to initialize the second application. You can also use the +initialize_copy+
+  # on one of the applications to create a copy of the application which shares
+  # the configuration.
+  #
+  # If you decide to define rake tasks, runners, or initializers in an
+  # application other than +Rails.application+, then you must run those
+  # these manually.
   class Application < Engine
     autoload :Bootstrap,              'rails/application/bootstrap'
     autoload :Configuration,          'rails/application/configuration'
@@ -61,12 +84,16 @@ class Application < Engine
 
     class << self
       def inherited(base)
-        raise "You cannot have more than one Rails::Application" if Rails.application
         super
-        Rails.application = base.instance
-        Rails.application.add_lib_to_load_path!
-        ActiveSupport.run_load_hooks(:before_configuration, base.instance)
+        Rails.application ||= base.instance
       end
+
+      # Makes the +new+ method public.
+      #
+      # Note that Rails::Application inherits from Rails::Engine, which
+      # inherits from Rails::Railtie and the +new+ method on Rails::Railtie is
+      # private
+      public :new
     end
 
     attr_accessor :assets, :sandbox
@@ -75,14 +102,28 @@ def inherited(base)
 
     delegate :default_url_options, :default_url_options=, to: :routes
 
-    def initialize
-      super
+    INITIAL_VARIABLES = [:config, :railties, :routes_reloader, :reloaders,
+                         :routes, :helpers, :app_env_config] # :nodoc:
+
+    def initialize(initial_variable_values = {}, &block)
+      super()
       @initialized      = false
       @reloaders        = []
       @routes_reloader  = nil
       @app_env_config   = nil
       @ordered_railties = nil
       @railties         = nil
+
+      add_lib_to_load_path!
+      ActiveSupport.run_load_hooks(:before_configuration, self)
+
+      initial_variable_values.each do |variable_name, value|
+        if INITIAL_VARIABLES.include?(variable_name)
+          instance_variable_set("@#{variable_name}", value)
+        end
+      end
+
+      instance_eval(&block) if block_given?
     end
 
     # Returns true if the application is initialized.
@@ -141,6 +182,30 @@ def env_config
       end
     end
 
+    # If you try to define a set of rake tasks on the instance, these will get
+    # passed up to the rake tasks defined on the application's class.
+    def rake_tasks(&block)
+      self.class.rake_tasks(&block)
+    end
+
+    # Sends the initializers to the +initializer+ method defined in the
+    # Rails::Initializable module. Each Rails::Application class has its own
+    # set of initializers, as defined by the Initializable module.
+    def initializer(name, opts={}, &block)
+      self.class.initializer(name, opts, &block)
+    end
+
+    # Sends any runner called in the instance of a new application up
+    # to the +runner+ method defined in Rails::Railtie.
+    def runner(&blk)
+      self.class.runner(&blk)
+    end
+
+    # Sends the +isolate_namespace+ method up to the class method.
+    def isolate_namespace(mod)
+      self.class.isolate_namespace(mod)
+    end
+
     ## Rails internal API
 
     # This method is called just after an application inherits from Rails::Application,
@@ -158,7 +223,9 @@ def env_config
     # you need to load files in lib/ during the application configuration as well.
     def add_lib_to_load_path! #:nodoc:
       path = File.join config.root, 'lib'
-      $LOAD_PATH.unshift(path) if File.exists?(path)
+      if File.exists?(path) && !$LOAD_PATH.include?(path)
+        $LOAD_PATH.unshift(path)
+      end
     end
 
     def require_environment! #:nodoc:
@@ -202,6 +269,10 @@ def config #:nodoc:
       @config ||= Application::Configuration.new(find_root_with_flag("config.ru", Dir.pwd))
     end
 
+    def config=(configuration) #:nodoc:
+      @config = configuration
+    end
+
     def to_app #:nodoc:
       self
     end
diff --git a/railties/test/application/multiple_applications_test.rb b/railties/test/application/multiple_applications_test.rb
new file mode 100644
index 0000000000000..03c343c4751e6
--- /dev/null
+++ b/railties/test/application/multiple_applications_test.rb
@@ -0,0 +1,148 @@
+require 'isolation/abstract_unit'
+
+module ApplicationTests
+  class MultipleApplicationsTest < ActiveSupport::TestCase
+    include ActiveSupport::Testing::Isolation
+
+    def setup
+      build_app(initializers: true)
+      boot_rails
+      require "#{rails_root}/config/environment"
+    end
+
+    def teardown
+      teardown_app
+    end
+
+    def test_cloning_an_application_makes_a_shallow_copy_of_config
+      clone = Rails.application.clone
+
+      assert_equal Rails.application.config, clone.config, "The cloned application should get a copy of the config"
+      assert_equal Rails.application.config.secret_key_base, clone.config.secret_key_base, "The base secret key on the config should be the same"
+    end
+
+    def test_initialization_of_multiple_copies_of_same_application
+      application1 = AppTemplate::Application.new
+      application2 = AppTemplate::Application.new
+
+      assert_not_equal Rails.application.object_id, application1.object_id, "New applications should not be the same as the original application"
+      assert_not_equal Rails.application.object_id, application2.object_id, "New applications should not be the same as the original application"
+    end
+
+    def test_initialization_of_application_with_previous_config
+      application1 = AppTemplate::Application.new(config: Rails.application.config)
+      application2 = AppTemplate::Application.new
+
+      assert_equal Rails.application.config, application1.config, "Creating a new application while setting an initial config should result in the same config"
+      assert_not_equal Rails.application.config, application2.config, "New applications without setting an initial config should not have the same config"
+    end
+
+    def test_initialization_of_application_with_previous_railties
+      application1 = AppTemplate::Application.new(railties: Rails.application.railties)
+      application2 = AppTemplate::Application.new
+
+      assert_equal Rails.application.railties, application1.railties
+      assert_not_equal Rails.application.railties, application2.railties
+    end
+
+    def test_initialize_new_application_with_all_previous_initialization_variables
+      application1 = AppTemplate::Application.new(
+        config:           Rails.application.config,
+        railties:         Rails.application.railties,
+        routes_reloader:  Rails.application.routes_reloader,
+        reloaders:        Rails.application.reloaders,
+        routes:           Rails.application.routes,
+        helpers:          Rails.application.helpers,
+        app_env_config:   Rails.application.env_config
+      )
+
+      assert_equal Rails.application.config, application1.config
+      assert_equal Rails.application.railties, application1.railties
+      assert_equal Rails.application.routes_reloader, application1.routes_reloader
+      assert_equal Rails.application.reloaders, application1.reloaders
+      assert_equal Rails.application.routes, application1.routes
+      assert_equal Rails.application.helpers, application1.helpers
+      assert_equal Rails.application.env_config, application1.env_config
+    end
+
+    def test_rake_tasks_defined_on_different_applications_go_to_the_same_class
+      $run_count = 0
+
+      application1 = AppTemplate::Application.new
+      application1.rake_tasks do
+        $run_count += 1
+      end
+
+      application2 = AppTemplate::Application.new
+      application2.rake_tasks do
+        $run_count += 1
+      end
+
+      require "#{app_path}/config/environment"
+
+      assert_equal 0, $run_count, "The count should stay at zero without any calls to the rake tasks"
+      require 'rake'
+      require 'rake/testtask'
+      require 'rdoc/task'
+      Rails.application.load_tasks
+      assert_equal 2, $run_count, "Calling a rake task should result in two increments to the count"
+    end
+
+    def test_multiple_applications_can_be_initialized
+      assert_nothing_raised { AppTemplate::Application.new }
+    end
+
+    def test_initializers_run_on_different_applications_go_to_the_same_class
+      application1 = AppTemplate::Application.new
+      $run_count = 0
+
+      AppTemplate::Application.initializer :init0 do
+        $run_count += 1
+      end
+
+      application1.initializer :init1 do
+        $run_count += 1
+      end
+
+      AppTemplate::Application.new.initializer :init2 do
+        $run_count += 1
+      end
+
+      assert_equal 0, $run_count, "Without loading the initializers, the count should be 0"
+
+      # Set config.eager_load to false so that a eager_load warning doesn't pop up
+      AppTemplate::Application.new { config.eager_load = false }.initialize!
+
+      assert_equal 3, $run_count, "There should have been three initializers that incremented the count"
+    end
+
+    def test_runners_run_on_different_applications_go_to_the_same_class
+      $run_count = 0
+      AppTemplate::Application.runner { $run_count += 1 }
+      AppTemplate::Application.new.runner { $run_count += 1 }
+
+      assert_equal 0, $run_count, "Without loading the runners, the count should be 0"
+      Rails.application.load_runner
+      assert_equal 2, $run_count, "There should have been two runners that increment the count"
+    end
+
+    def test_isolate_namespace_on_an_application
+      assert_nil Rails.application.railtie_namespace, "Before isolating namespace, the railtie namespace should be nil"
+      Rails.application.isolate_namespace(AppTemplate)
+      assert_equal Rails.application.railtie_namespace, AppTemplate, "After isolating namespace, we should have a namespace"
+    end
+
+    def test_inserting_configuration_into_application
+      app = AppTemplate::Application.new(config: Rails.application.config)
+      new_config = Rails::Application::Configuration.new("root_of_application")
+      new_config.secret_key_base = "some_secret_key_dude"
+      app.config.secret_key_base = "a_different_secret_key"
+
+      assert_equal "a_different_secret_key", app.config.secret_key_base, "The configuration's secret key should be set."
+      app.config = new_config
+      assert_equal "some_secret_key_dude", app.config.secret_key_base, "The configuration's secret key should have changed."
+      assert_equal "root_of_application", app.config.root, "The root should have changed to the new config's root."
+      assert_equal new_config, app.config, "The application's config should have changed to the new config."
+    end
+  end
+end