diff --git a/activemodel/CHANGELOG b/activemodel/CHANGELOG
new file mode 100644
index 0000000000000..142038cc87354
--- /dev/null
+++ b/activemodel/CHANGELOG
@@ -0,0 +1,5 @@
+*Edge*
+
+* Introduce validates_with to encapsulate attribute validations in a class.  #2630 [Jeff Dean]
+
+* Extracted from Active Record and Active Resource.
diff --git a/activemodel/CHANGES b/activemodel/CHANGES
index a9f9c275076c6..217a6d6bf7dad 100644
--- a/activemodel/CHANGES
+++ b/activemodel/CHANGES
@@ -9,4 +9,4 @@ Changes from extracting bits to ActiveModel
 	    klass.add_observer(self)
 	    klass.class_eval 'def after_find() end' unless 
 				klass.respond_to?(:after_find)
-	  end
\ No newline at end of file
+	  end
diff --git a/activemodel/lib/active_model/validations/with.rb b/activemodel/lib/active_model/validations/with.rb
new file mode 100644
index 0000000000000..851cdfebf0d8e
--- /dev/null
+++ b/activemodel/lib/active_model/validations/with.rb
@@ -0,0 +1,64 @@
+module ActiveModel
+  module Validations
+    module ClassMethods
+
+      # Passes the record off to the class or classes specified and allows them to add errors based on more complex conditions.
+      #
+      #   class Person < ActiveRecord::Base
+      #     validates_with MyValidator
+      #   end
+      #
+      #   class MyValidator < ActiveRecord::Validator
+      #     def validate
+      #       if some_complex_logic
+      #         record.errors[:base] << "This record is invalid"
+      #       end
+      #     end
+      #
+      #     private
+      #       def some_complex_logic
+      #         # ...
+      #       end
+      #   end
+      #
+      # You may also pass it multiple classes, like so:
+      #
+      #   class Person < ActiveRecord::Base
+      #     validates_with MyValidator, MyOtherValidator, :on => :create
+      #   end
+      #
+      # Configuration options:
+      # * <tt>on</tt> - Specifies when this validation is active (<tt>:create</tt> or <tt>:update</tt>
+      # * <tt>if</tt> - Specifies a method, proc or string to call to determine if the validation should
+      #   occur (e.g. <tt>:if => :allow_validation</tt>, or <tt>:if => Proc.new { |user| user.signup_step > 2 }</tt>).
+      #   The method, proc or string should return or evaluate to a true or false value.
+      # * <tt>unless</tt> - Specifies a method, proc or string to call to determine if the validation should
+      #   not occur (e.g. <tt>:unless => :skip_validation</tt>, or <tt>:unless => Proc.new { |user| user.signup_step <= 2 }</tt>).
+      #   The method, proc or string should return or evaluate to a true or false value.
+      #
+      # If you pass any additional configuration options, they will be passed to the class and available as <tt>options</tt>:
+      #
+      #   class Person < ActiveRecord::Base
+      #     validates_with MyValidator, :my_custom_key => "my custom value"
+      #   end
+      #
+      #   class MyValidator < ActiveRecord::Validator
+      #     def validate
+      #       options[:my_custom_key] # => "my custom value"
+      #     end
+      #   end
+      #
+      def validates_with(*args)
+        configuration = args.extract_options!
+
+        send(validation_method(configuration[:on]), configuration) do |record|
+          args.each do |klass|
+            klass.new(record, configuration.except(:on, :if, :unless)).validate
+          end
+        end
+      end
+    end
+  end
+end
+
+
diff --git a/activemodel/test/cases/validations/with_validation_test.rb b/activemodel/test/cases/validations/with_validation_test.rb
new file mode 100644
index 0000000000000..f55fdc5864588
--- /dev/null
+++ b/activemodel/test/cases/validations/with_validation_test.rb
@@ -0,0 +1,116 @@
+# encoding: utf-8
+require 'cases/helper'
+
+require 'models/topic'
+
+class ValidatesWithTest < ActiveRecord::TestCase
+  include ActiveModel::ValidationsRepairHelper
+
+  repair_validations(Topic)
+
+  ERROR_MESSAGE = "Validation error from validator"
+  OTHER_ERROR_MESSAGE = "Validation error from other validator"
+
+  class ValidatorThatAddsErrors < ActiveRecord::Validator
+    def validate()
+      record.errors[:base] << ERROR_MESSAGE
+    end
+  end
+
+  class OtherValidatorThatAddsErrors < ActiveRecord::Validator
+    def validate()
+      record.errors[:base] << OTHER_ERROR_MESSAGE
+    end
+  end
+
+  class ValidatorThatDoesNotAddErrors < ActiveRecord::Validator
+    def validate()
+    end
+  end
+
+  class ValidatorThatValidatesOptions < ActiveRecord::Validator
+    def validate()
+      if options[:field] == :first_name
+        record.errors[:base] << ERROR_MESSAGE
+      end
+    end
+  end
+
+  test "vaidation with class that adds errors" do
+    Topic.validates_with(ValidatorThatAddsErrors)
+    topic = Topic.new
+    assert !topic.valid?, "A class that adds errors causes the record to be invalid"
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+  end
+
+  test "with a class that returns valid" do
+    Topic.validates_with(ValidatorThatDoesNotAddErrors)
+    topic = Topic.new
+    assert topic.valid?, "A class that does not add errors does not cause the record to be invalid"
+  end
+
+  test "with a class that adds errors on update and a new record" do
+    Topic.validates_with(ValidatorThatAddsErrors, :on => :update)
+    topic = Topic.new
+    assert topic.valid?, "Validation doesn't run on create if 'on' is set to update"
+  end
+
+  test "with a class that adds errors on create and a new record" do
+    Topic.validates_with(ValidatorThatAddsErrors, :on => :create)
+    topic = Topic.new
+    assert !topic.valid?, "Validation does run on create if 'on' is set to create"
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+  end
+
+  test "with multiple classes" do
+    Topic.validates_with(ValidatorThatAddsErrors, OtherValidatorThatAddsErrors)
+    topic = Topic.new
+    assert !topic.valid?
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+    assert topic.errors[:base].include?(OTHER_ERROR_MESSAGE)
+  end
+
+  test "with if statements that return false" do
+    Topic.validates_with(ValidatorThatAddsErrors, :if => "1 == 2")
+    topic = Topic.new
+    assert topic.valid?
+  end
+
+  test "with if statements that return true" do
+    Topic.validates_with(ValidatorThatAddsErrors, :if => "1 == 1")
+    topic = Topic.new
+    assert !topic.valid?
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+  end
+
+  test "with unless statements that return true" do
+    Topic.validates_with(ValidatorThatAddsErrors, :unless => "1 == 1")
+    topic = Topic.new
+    assert topic.valid?
+  end
+
+  test "with unless statements that returns false" do
+    Topic.validates_with(ValidatorThatAddsErrors, :unless => "1 == 2")
+    topic = Topic.new
+    assert !topic.valid?
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+  end
+
+  test "passes all non-standard configuration options to the validator class" do
+    topic = Topic.new
+    validator = mock()
+    validator.expects(:new).with(topic, {:foo => :bar}).returns(validator)
+    validator.expects(:validate)
+
+    Topic.validates_with(validator, :if => "1 == 1", :foo => :bar)
+    assert topic.valid?
+  end
+
+  test "validates_with with options" do
+    Topic.validates_with(ValidatorThatValidatesOptions, :field => :first_name)
+    topic = Topic.new
+    assert !topic.valid?
+    assert topic.errors[:base].include?(ERROR_MESSAGE)
+  end
+
+end
diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index 68b025198234d..2f1e7573d8fab 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -69,6 +69,7 @@ def self.load_all!
   autoload :TestCase, 'active_record/test_case'
   autoload :Timestamp, 'active_record/timestamp'
   autoload :Transactions, 'active_record/transactions'
+  autoload :Validator, 'active_record/validator'
   autoload :Validations, 'active_record/validations'
 
   module AttributeMethods
diff --git a/activerecord/lib/active_record/validator.rb b/activerecord/lib/active_record/validator.rb
new file mode 100644
index 0000000000000..83a33f4dcda64
--- /dev/null
+++ b/activerecord/lib/active_record/validator.rb
@@ -0,0 +1,68 @@
+module ActiveRecord #:nodoc:
+
+  # A simple base class that can be used along with ActiveRecord::Base.validates_with
+  #
+  #   class Person < ActiveRecord::Base
+  #     validates_with MyValidator
+  #   end
+  #
+  #   class MyValidator < ActiveRecord::Validator
+  #     def validate
+  #       if some_complex_logic
+  #         record.errors[:base] = "This record is invalid"
+  #       end
+  #     end
+  #
+  #     private
+  #       def some_complex_logic
+  #         # ...
+  #       end
+  #   end
+  #
+  # Any class that inherits from ActiveRecord::Validator will have access to <tt>record</tt>,
+  # which is an instance of the record being validated, and must implement a method called <tt>validate</tt>.
+  #
+  #   class Person < ActiveRecord::Base
+  #     validates_with MyValidator
+  #   end
+  #
+  #   class MyValidator < ActiveRecord::Validator
+  #     def validate
+  #       record # => The person instance being validated
+  #       options # => Any non-standard options passed to validates_with
+  #     end
+  #   end
+  #
+  # To cause a validation error, you must add to the <tt>record<tt>'s errors directly
+  # from within the validators message
+  #
+  #   class MyValidator < ActiveRecord::Validator
+  #     def validate
+  #       record.errors[:base] << "This is some custom error message"
+  #       record.errors[:first_name] << "This is some complex validation"
+  #       # etc...
+  #     end
+  #   end
+  #
+  # To add behavior to the initialize method, use the following signature:
+  #
+  #   class MyValidator < ActiveRecord::Validator
+  #     def initialize(record, options)
+  #       super
+  #       @my_custom_field = options[:field_name] || :first_name
+  #     end
+  #   end
+  #
+  class Validator
+    attr_reader :record, :options
+
+    def initialize(record, options)
+      @record = record
+      @options = options
+    end
+
+    def validate
+      raise "You must override this method"
+    end
+  end
+end