diff --git a/.gitignore b/.gitignore
index b3bf3d1..078ecca 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,4 +16,4 @@ TODO
 alto
 .rspec
 .bundle
-
+tags
diff --git a/lib/aasm/event.rb b/lib/aasm/event.rb
index c62da51..05b3198 100644
--- a/lib/aasm/event.rb
+++ b/lib/aasm/event.rb
@@ -6,7 +6,8 @@ module AASM
     def initialize(name, options = {}, &block)
       @name = name
       @transitions = []
-      @guards = Array(options[:guard] || options[:guards])
+      @guards = Array(options[:guard] || options[:guards] || options[:if])
+      @unless = Array(options[:unless]) #TODO: This could use a better name
       update(options, &block)
     end
 
@@ -74,9 +75,13 @@ module AASM
 
     def attach_event_guards(definitions)
       unless @guards.empty?
-        given_guards = Array(definitions.delete(:guard) || definitions.delete(:guards))
+        given_guards = Array(definitions.delete(:guard) || definitions.delete(:guards) || definitions.delete(:if))
         definitions[:guards] = given_guards + @guards
       end
+      unless @unless.empty?
+        given_unless = Array(definitions.delete(:unless))
+        definitions[:unless] = given_unless + @unless
+      end
       definitions
     end
 
@@ -100,7 +105,7 @@ module AASM
 
       transitions.each do |transition|
         next if to_state and !Array(transition.to).include?(to_state)
-        if transition.perform(obj, *args)
+        if transition.allowed?(obj, *args)
           if test
             result = true
           else
diff --git a/lib/aasm/transition.rb b/lib/aasm/transition.rb
index 88c4dc2..e682fa8 100644
--- a/lib/aasm/transition.rb
+++ b/lib/aasm/transition.rb
@@ -6,13 +6,14 @@ module AASM
     def initialize(opts)
       @from = opts[:from]
       @to = opts[:to]
-      @guards = Array(opts[:guard] || opts[:guards])
+      @guards = Array(opts[:guard] || opts[:guards] || opts[:if])
+      @unless = Array(opts[:unless]) #TODO: This could use a better name
       @on_transition = opts[:on_transition]
       @opts = opts
     end
 
     # TODO: should be named allowed? or similar
-    def perform(obj, *args)
+    def allowed?(obj, *args)
       @guards.each do |guard|
         case guard
         when Symbol, String
@@ -21,6 +22,15 @@ module AASM
           return false unless guard.call(obj, *args)
         end
       end
+
+      @unless.each do |guard|
+        case guard
+        when Symbol, String
+          return false if obj.send(guard, *args)
+        when Proc
+          return false if guard.call(obj, *args)
+        end
+      end
       true
     end
 
diff --git a/spec/unit/transition_spec.rb b/spec/unit/transition_spec.rb
index 87fe791..6610ac6 100644
--- a/spec/unit/transition_spec.rb
+++ b/spec/unit/transition_spec.rb
@@ -1,4 +1,5 @@
 require 'spec_helper'
+require 'byebug'
 
 describe 'transitions' do
 
@@ -100,7 +101,7 @@ describe AASM::Transition, '- when performing guard checks' do
     opts = {:from => 'foo', :to => 'bar'}
     st = AASM::Transition.new(opts)
 
-    expect(st.perform(nil)).to be_true
+    expect(st.allowed?(nil)).to be_true
   end
 
   it 'should call the method on the object if guard is a symbol' do
@@ -110,7 +111,17 @@ describe AASM::Transition, '- when performing guard checks' do
     obj = double('object')
     expect(obj).to receive(:test)
 
-    st.perform(obj)
+    expect(st.allowed?(obj)).to be false
+  end
+
+  it 'should call the method on the object if unless is a symbol' do
+    opts = {:from => 'foo', :to => 'bar', :unless => :test}
+    st = AASM::Transition.new(opts)
+
+    obj = double('object')
+    expect(obj).to receive(:test)
+
+    expect(st.allowed?(obj)).to be true
   end
 
   it 'should call the method on the object if guard is a string' do
@@ -120,7 +131,17 @@ describe AASM::Transition, '- when performing guard checks' do
     obj = double('object')
     expect(obj).to receive(:test)
 
-    st.perform(obj)
+    expect(st.allowed?(obj)).to be false
+  end
+
+  it 'should call the method on the object if unless is a string' do
+    opts = {:from => 'foo', :to => 'bar', :unless => 'test'}
+    st = AASM::Transition.new(opts)
+
+    obj = double('object')
+    expect(obj).to receive(:test)
+
+    expect(st.allowed?(obj)).to be true
   end
 
   it 'should call the proc passing the object if the guard is a proc' do
@@ -130,7 +151,7 @@ describe AASM::Transition, '- when performing guard checks' do
     obj = double('object')
     expect(obj).to receive(:test)
 
-    st.perform(obj)
+    expect(st.allowed?(obj)).to be false
   end
 end