diff --git a/activesupport/lib/active_support/core_ext/class.rb b/activesupport/lib/active_support/core_ext/class.rb
index 62df7d8b82..f2ca9c7cc9 100644
--- a/activesupport/lib/active_support/core_ext/class.rb
+++ b/activesupport/lib/active_support/core_ext/class.rb
@@ -1,3 +1,4 @@
 require 'active_support/core_ext/class/attribute_accessors'
 require 'active_support/core_ext/class/inheritable_attributes'
 require 'active_support/core_ext/class/delegating_attributes'
+require 'active_support/core_ext/class/subclasses'
diff --git a/activesupport/lib/active_support/core_ext/class/subclasses.rb b/activesupport/lib/active_support/core_ext/class/subclasses.rb
new file mode 100644
index 0000000000..c166ce8079
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/class/subclasses.rb
@@ -0,0 +1,58 @@
+require 'active_support/core_ext/object/blank'
+
+class Class #:nodoc:
+  # Returns an array with the names of the subclasses of +self+ as strings.
+  #
+  #   Integer.subclasses # => ["Bignum", "Fixnum"]
+  def subclasses
+    Class.subclasses_of(self).map { |o| o.to_s }
+  end
+
+  def reachable? #:nodoc:
+    eval("defined?(::#{self}) && ::#{self}.equal?(self)")
+  end
+
+  # Rubinius
+  if defined?(Class.__subclasses__)
+    def descendents
+      subclasses = []
+      __subclasses__.each {|k| subclasses << k; subclasses.concat k.descendents }
+      subclasses
+    end
+  else
+    # MRI
+    begin
+      ObjectSpace.each_object(Class.new) {}
+
+      def descendents
+        subclasses = []
+        ObjectSpace.each_object(class << self; self; end) do |k|
+          subclasses << k unless k == self
+        end
+        subclasses
+      end
+    # JRuby
+    rescue StandardError
+      def descendents
+        subclasses = []
+        ObjectSpace.each_object(Class) do |k|
+          subclasses << k if k < self
+        end
+        subclasses.uniq!
+        subclasses
+      end
+    end
+  end
+
+  # Exclude this class unless it's a subclass of our supers and is defined.
+  # We check defined? in case we find a removed class that has yet to be
+  # garbage collected. This also fails for anonymous classes -- please
+  # submit a patch if you have a workaround.
+  def self.subclasses_of(*superclasses) #:nodoc:
+    subclasses = []
+    superclasses.each do |klass|
+      subclasses.concat klass.descendents.select {|k| k.name.blank? || k.reachable?}
+    end
+    subclasses
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/object/extending.rb b/activesupport/lib/active_support/core_ext/object/extending.rb
new file mode 100644
index 0000000000..c4c37b6a2a
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/object/extending.rb
@@ -0,0 +1,11 @@
+require 'active_support/core_ext/class/subclasses'
+
+class Object
+  # Exclude this class unless it's a subclass of our supers and is defined.
+  # We check defined? in case we find a removed class that has yet to be
+  # garbage collected. This also fails for anonymous classes -- please
+  # submit a patch if you have a workaround.
+  def subclasses_of(*superclasses) #:nodoc:
+    Class.subclasses_of(*superclasses)
+  end
+end
diff --git a/activesupport/test/core_ext/class_test.rb b/activesupport/test/core_ext/class_test.rb
new file mode 100644
index 0000000000..b7f3dd9930
--- /dev/null
+++ b/activesupport/test/core_ext/class_test.rb
@@ -0,0 +1,29 @@
+require 'abstract_unit'
+require 'active_support/core_ext/class'
+
+class A
+end
+
+module X
+  class B
+  end
+end
+
+module Y
+  module Z
+    class C
+    end
+  end
+end
+
+class ClassTest < Test::Unit::TestCase
+  def test_retrieving_subclasses
+    @parent   = eval("class D; end; D")
+    @sub      = eval("class E < D; end; E")
+    @subofsub = eval("class F < E; end; F")
+    assert_equal 2, @parent.subclasses.size
+    assert_equal [@subofsub.to_s], @sub.subclasses
+    assert_equal [], @subofsub.subclasses
+    assert_equal [@sub.to_s, @subofsub.to_s].sort, @parent.subclasses.sort
+  end
+end
diff --git a/activesupport/test/core_ext/object_and_class_ext_test.rb b/activesupport/test/core_ext/object_and_class_ext_test.rb
index 0b2a9c418e..f31e7774e9 100644
--- a/activesupport/test/core_ext/object_and_class_ext_test.rb
+++ b/activesupport/test/core_ext/object_and_class_ext_test.rb
@@ -1,6 +1,7 @@
 require 'abstract_unit'
 require 'active_support/time'
 require 'active_support/core_ext/object'
+require 'active_support/core_ext/class/subclasses'
 
 class ClassA; end
 class ClassB < ClassA; end
@@ -39,6 +40,55 @@ class Foo
   include Bar
 end
 
+class ClassExtTest < Test::Unit::TestCase
+  def test_subclasses_of_should_find_nested_classes
+    assert Class.subclasses_of(ClassK).include?(Nested::ClassL)
+  end
+
+  def test_subclasses_of_should_not_return_removed_classes
+    # First create the removed class
+    old_class = Nested.class_eval { remove_const :ClassL }
+    new_class = Class.new(ClassK)
+    Nested.const_set :ClassL, new_class
+    assert_equal "Nested::ClassL", new_class.name # Sanity check
+
+    subclasses = Class.subclasses_of(ClassK)
+    assert subclasses.include?(new_class)
+    assert ! subclasses.include?(old_class)
+  ensure
+    Nested.const_set :ClassL, old_class unless defined?(Nested::ClassL)
+  end
+
+  def test_subclasses_of_should_not_trigger_const_missing
+    const_missing = false
+    Nested.on_const_missing { const_missing = true }
+
+    subclasses = Class.subclasses_of ClassK
+    assert !const_missing
+    assert_equal [ Nested::ClassL ], subclasses
+
+    removed = Nested.class_eval { remove_const :ClassL }  # keep it in memory
+    subclasses = Class.subclasses_of ClassK
+    assert !const_missing
+    assert subclasses.empty?
+  ensure
+    Nested.const_set :ClassL, removed unless defined?(Nested::ClassL)
+  end
+
+  def test_subclasses_of_with_multiple_roots
+    classes = Class.subclasses_of(ClassI, ClassK)
+    assert_equal %w(ClassJ Nested::ClassL), classes.collect(&:to_s).sort
+  end
+
+  def test_subclasses_of_doesnt_find_anonymous_classes
+    assert_equal [], Class.subclasses_of(Foo)
+    bar = Class.new(Foo)
+    assert_nothing_raised do
+      assert_equal [bar], Class.subclasses_of(Foo)
+    end
+  end
+end
+
 class ObjectTests < Test::Unit::TestCase
   class DuckTime
     def acts_like_time?