diff --git a/lib/vagrant/environment.rb b/lib/vagrant/environment.rb
index be5774511..eaed02cea 100644
--- a/lib/vagrant/environment.rb
+++ b/lib/vagrant/environment.rb
@@ -100,6 +100,9 @@ module Vagrant
 
       # Load the plugins
       load_plugins
+
+      # Activate the plugins
+      activate_plugins
     end
 
     #---------------------------------------------------------------
@@ -395,6 +398,10 @@ module Vagrant
           config_loader.set(:vm, subvm.proc_stack)
         end
 
+        # We activate plugins here because the files which we're loading
+        # configuration from may have defined new plugins as well.
+        activate_plugins
+
         # Execute the configuration stack and store the result as the final
         # value in the config ivar.
         config_loader.load
@@ -515,6 +522,14 @@ module Vagrant
       nil
     end
 
+    # This finds all the current plugins and activates them. This is an
+    # idempotent call so it is safe to call this as much as you need.
+    def activate_plugins
+      Vagrant.plugin("1").registered.each do |plugin|
+        plugin.activate!
+      end
+    end
+
     # Loads the Vagrant plugins by properly setting up RubyGems so that
     # our private gem repository is on the path.
     def load_plugins
diff --git a/lib/vagrant/plugin/v1.rb b/lib/vagrant/plugin/v1.rb
index 30e9760df..1030d80da 100644
--- a/lib/vagrant/plugin/v1.rb
+++ b/lib/vagrant/plugin/v1.rb
@@ -72,6 +72,19 @@ module Vagrant
         hooks << block
       end
 
+      # The given block will be called when this plugin is activated. The
+      # activation block should be used to load any of the classes used by
+      # the plugin other than the plugin definition itself. Vagrant only
+      # guarantees that the plugin definition will remain backwards
+      # compatible, but not any other classes (such as command base classes
+      # or so on). Therefore, to avoid crashing future versions of Vagrant,
+      # these classes should be placed in separate files and loaded in the
+      # activation block.
+      def self.activated(&block)
+        data[:activation_block] = block if block_given?
+        data[:activation_block]
+      end
+
       # Defines additional command line commands available by key. The key
       # becomes the subcommand, so if you register a command "foo" then
       # "vagrant foo" becomes available.
@@ -185,6 +198,18 @@ module Vagrant
         data[:provisioners]
       end
 
+      # Activates the current plugin. This shouldn't be called publicly.
+      def self.activate!
+        if !data[:activated_called]
+          LOGGER.info("Activating plugin: #{self.name}")
+          data[:activated_called] = true
+
+          # Call the activation block
+          block = activated
+          block.call if block
+        end
+      end
+
       # Registers the plugin. This makes the plugin actually work with
       # Vagrant. Prior to registering, the plugin is merely a skeleton.
       #
diff --git a/test/unit/vagrant/plugin/v1_test.rb b/test/unit/vagrant/plugin/v1_test.rb
index c187b3ab5..e54757064 100644
--- a/test/unit/vagrant/plugin/v1_test.rb
+++ b/test/unit/vagrant/plugin/v1_test.rb
@@ -35,6 +35,34 @@ describe Vagrant::Plugin::V1 do
     end
   end
 
+  describe "activation block" do
+    it "should have no activation block by default" do
+      plugin = Class.new(described_class)
+      plugin.activated.should be_nil
+    end
+
+    it "should be able to set and get the activation block" do
+      plugin = Class.new(described_class) do
+        activated do
+          42
+        end
+      end
+
+      plugin.activated.call.should == 42
+    end
+
+    it "should activate when `activate!` is called" do
+      plugin = Class.new(described_class) do
+        activated do
+          raise NotImplementedError
+        end
+      end
+
+      expect { plugin.activate! }.to raise_error(NotImplementedError)
+      expect { plugin.activate! }.to_not raise_error
+    end
+  end
+
   describe "commands" do
     it "should register command classes" do
       plugin = Class.new(described_class) do