diff --git a/lib/immutable-struct.rb b/lib/immutable-struct.rb
index 31d09e9..b914461 100644
--- a/lib/immutable-struct.rb
+++ b/lib/immutable-struct.rb
@@ -82,13 +82,18 @@ def self.new(*attributes,&block)
       end
 
       alias_method :eql?, :==
+
+      define_method(:hash) do
+        attribute_values = attributes.map { |attr| self.send(attr) }
+        (attribute_values + [self.class]).hash
+      end
     end
     klass.class_exec(&block) unless block.nil?
     imethods = klass.instance_methods(include_super=false)
     klass.class_exec(imethods) do |imethods|
       define_method(:to_h) do
         imethods.inject({}) do |hash, method|
-          next hash if [:==, :eql?, :merge].include?(method)
+          next hash if [:==, :eql?, :merge, :hash].include?(method)
           hash.merge(method.to_sym => self.send(method))
         end
       end
diff --git a/spec/immutable_struct_spec.rb b/spec/immutable_struct_spec.rb
index 67801ab..854961f 100644
--- a/spec/immutable_struct_spec.rb
+++ b/spec/immutable_struct_spec.rb
@@ -192,5 +192,45 @@ def lawsuit
 
     end
 
+    describe "hash" do
+
+      it "should have same hash value as itself" do
+        @k1_a.hash.eql?(@k1_a.hash).should be true
+      end
+
+      it "should have same hash value as same class with identical attribute values" do
+        @k1_a.hash.eql?(@k1_c.hash).should be true
+      end
+
+      it 'should not have hash value as same class with different attribute values' do
+        @k1_a.hash.eql?(@k1_b.hash).should be false
+      end
+
+      it 'should not have hash value equal to different class with identical attribute values' do
+        @k1_a.hash.eql?(@k3_a.hash).should be false
+      end
+
+      it 'should reject set addition if same instance is already a member' do
+        set = Set.new([@k1_a])
+        set.add?(@k1_a).should be nil
+      end
+
+      it 'should reject set addition if different instance, but attributes are the same' do
+        set = Set.new([@k1_a])
+        set.add?(@k1_c).should be nil
+      end
+
+      it 'should allow set addition if different instance and attribute values' do
+        set = Set.new([@k1_a])
+        set.add?(@k1_b).should_not be nil
+      end
+
+      it 'should allow set addition if different class' do
+        set = Set.new([@k1_a])
+        set.add?(@k2_a).should_not be nil
+      end
+
+    end
+
   end
 end