diff --git a/activemodel/lib/active_model/observer_array.rb b/activemodel/lib/active_model/observer_array.rb
index f3b5811b81..d501215dd6 100644
--- a/activemodel/lib/active_model/observer_array.rb
+++ b/activemodel/lib/active_model/observer_array.rb
@@ -4,15 +4,6 @@ module ActiveModel
   # Stores the enabled/disabled state of individual observers for
   # a particular model classes.
   class ObserverArray < Array
-    INSTANCES = Hash.new do |hash, model_class|
-      hash[model_class] = new(model_class)
-    end
-
-    def self.for(model_class)
-      return nil unless model_class < ActiveModel::Observing
-      INSTANCES[model_class]
-    end
-
     # returns false if:
     #   - the ObserverArray for the given model's class has the given observer
     #     in its disabled_observers set.
@@ -22,7 +13,8 @@ module ActiveModel
       observer_class = observer.class
 
       loop do
-        break unless array = self.for(klass)
+        break unless klass.respond_to?(:observers)
+        array = klass.observers
         return false if array.disabled_observers.include?(observer_class)
         klass = klass.superclass
       end
@@ -90,8 +82,8 @@ module ActiveModel
       end
 
       def each_subclass_array
-        model_class.subclasses.each do |subclass|
-          yield self.class.for(subclass)
+        model_class.descendants.each do |subclass|
+          yield subclass.observers
         end
       end
 
@@ -102,7 +94,7 @@ module ActiveModel
             yield
           end
         else
-          observers = ActiveModel::Observer.all_observers if observers == [:all]
+          observers = ActiveModel::Observer.descendants if observers == [:all]
           observers.each do |obs|
             klass = observer_class_for(obs)
 
diff --git a/activemodel/lib/active_model/observing.rb b/activemodel/lib/active_model/observing.rb
index ba6be46670..9ffcda8dd7 100644
--- a/activemodel/lib/active_model/observing.rb
+++ b/activemodel/lib/active_model/observing.rb
@@ -5,11 +5,16 @@ require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/module/remove_method'
 require 'active_support/core_ext/string/inflections'
 require 'active_support/core_ext/enumerable'
+require 'active_support/descendants_tracker'
 
 module ActiveModel
   module Observing
     extend ActiveSupport::Concern
 
+    included do
+      extend ActiveSupport::DescendantsTracker
+    end
+
     module ClassMethods
       # == Active Model Observers Activation
       #
@@ -37,7 +42,7 @@ module ActiveModel
 
       # Gets the current observers.
       def observers
-        @observers ||= ObserverArray.for(self)
+        @observers ||= ObserverArray.new(self)
       end
 
       # Gets the current observer instances.
@@ -70,10 +75,6 @@ module ActiveModel
         observer_instances.size
       end
 
-      def subclasses
-        @subclasses ||= []
-      end
-
       protected
         def instantiate_observer(observer) #:nodoc:
           # string/symbol
@@ -93,7 +94,6 @@ module ActiveModel
         # Notify observers when the observed class is subclassed.
         def inherited(subclass)
           super
-          subclasses << subclass
           notify_observers :observed_class_inherited, subclass
         end
     end
@@ -176,6 +176,7 @@ module ActiveModel
   #
   class Observer
     include Singleton
+    extend ActiveSupport::DescendantsTracker
 
     class << self
       # Attaches the observer to the supplied model classes.
@@ -208,23 +209,6 @@ module ActiveModel
           nil
         end
       end
-
-      def subclasses
-        @subclasses ||= []
-      end
-
-      # List of all observer subclasses, sub-subclasses, etc.
-      # Necessary so we can disable or enable all observers.
-      def all_observers
-        subclasses.each_with_object(subclasses.dup) do |subclass, array|
-          array.concat(subclass.all_observers)
-        end
-      end
-    end
-
-    def self.inherited(subclass)
-      subclasses << subclass
-      super
     end
 
     # Start observing the declared classes and their subclasses.