diff --git a/packer/build_test.go b/packer/build_test.go
index c5eedbc81..bb990e2fa 100644
--- a/packer/build_test.go
+++ b/packer/build_test.go
@@ -13,7 +13,7 @@ func testBuild() *coreBuild {
 		builderConfig: 42,
 		builderType:   "foo",
 		hooks: map[string][]Hook{
-			"foo": []Hook{&TestHook{}},
+			"foo": []Hook{&MockHook{}},
 		},
 		provisioners: []coreBuildProvisioner{
 			coreBuildProvisioner{&TestProvisioner{}, []interface{}{42}},
@@ -187,9 +187,9 @@ func TestBuild_Run(t *testing.T) {
 	dispatchHook := builder.runHook
 	dispatchHook.Run("foo", nil, nil, 42)
 
-	hook := build.hooks["foo"][0].(*TestHook)
-	assert.True(hook.runCalled, "run should be called")
-	assert.Equal(hook.runData, 42, "should have correct data")
+	hook := build.hooks["foo"][0].(*MockHook)
+	assert.True(hook.RunCalled, "run should be called")
+	assert.Equal(hook.RunData, 42, "should have correct data")
 
 	// Verify provisioners run
 	dispatchHook.Run(HookProvision, nil, nil, 42)
diff --git a/packer/environment_test.go b/packer/environment_test.go
index e301e8c7a..d357797b4 100644
--- a/packer/environment_test.go
+++ b/packer/environment_test.go
@@ -227,7 +227,7 @@ func TestEnvironment_DefaultCli_Version(t *testing.T) {
 func TestEnvironment_Hook(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
-	hook := &TestHook{}
+	hook := &MockHook{}
 	hooks := make(map[string]Hook)
 	hooks["foo"] = hook
 
diff --git a/packer/hook.go b/packer/hook.go
index 4a79222f4..f31b62a5d 100644
--- a/packer/hook.go
+++ b/packer/hook.go
@@ -11,8 +11,14 @@ const HookProvision = "packer_provision"
 // you must reference the documentation for the specific hook you're interested
 // in. In addition to that, the Hook is given access to a UI so that it can
 // output things to the user.
+//
+// Cancel is called when the hook needs to be cancelled. This will usually
+// be called when Run is still in progress so the mechanism that handles this
+// must be race-free. Cancel should attempt to cancel the hook in the
+// quickest, safest way possible.
 type Hook interface {
 	Run(string, Ui, Communicator, interface{}) error
+	Cancel()
 }
 
 // A Hook implementation that dispatches based on an internal mapping.
@@ -38,3 +44,5 @@ func (h *DispatchHook) Run(name string, ui Ui, comm Communicator, data interface
 
 	return nil
 }
+
+func (h *DispatchHook) Cancel() {}
diff --git a/packer/hook_mock.go b/packer/hook_mock.go
new file mode 100644
index 000000000..eedf5bb90
--- /dev/null
+++ b/packer/hook_mock.go
@@ -0,0 +1,24 @@
+package packer
+
+// MockHook is an implementation of Hook that can be used for tests.
+type MockHook struct {
+	RunCalled    bool
+	RunComm      Communicator
+	RunData      interface{}
+	RunName      string
+	RunUi        Ui
+	CancelCalled bool
+}
+
+func (t *MockHook) Run(name string, ui Ui, comm Communicator, data interface{}) error {
+	t.RunCalled = true
+	t.RunComm = comm
+	t.RunData = data
+	t.RunName = name
+	t.RunUi = ui
+	return nil
+}
+
+func (t *MockHook) Cancel() {
+	t.CancelCalled = true
+}
diff --git a/packer/hook_test.go b/packer/hook_test.go
index e43dfd1fd..06db0073d 100644
--- a/packer/hook_test.go
+++ b/packer/hook_test.go
@@ -5,23 +5,6 @@ import (
 	"testing"
 )
 
-type TestHook struct {
-	runCalled bool
-	runComm   Communicator
-	runData   interface{}
-	runName   string
-	runUi     Ui
-}
-
-func (t *TestHook) Run(name string, ui Ui, comm Communicator, data interface{}) error {
-	t.runCalled = true
-	t.runComm = comm
-	t.runData = data
-	t.runName = name
-	t.runUi = ui
-	return nil
-}
-
 func TestDispatchHook_Implements(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
@@ -40,14 +23,14 @@ func TestDispatchHook_Run_NoHooks(t *testing.T) {
 func TestDispatchHook_Run(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
-	hook := &TestHook{}
+	hook := &MockHook{}
 
 	mapping := make(map[string][]Hook)
 	mapping["foo"] = []Hook{hook}
 	dh := &DispatchHook{mapping}
 	dh.Run("foo", nil, nil, 42)
 
-	assert.True(hook.runCalled, "run should be called")
-	assert.Equal(hook.runName, "foo", "should be proper event")
-	assert.Equal(hook.runData, 42, "should be correct data")
+	assert.True(hook.RunCalled, "run should be called")
+	assert.Equal(hook.RunName, "foo", "should be proper event")
+	assert.Equal(hook.RunData, 42, "should be correct data")
 }
diff --git a/packer/plugin/hook.go b/packer/plugin/hook.go
index 90b0779d9..5d4dbf06a 100644
--- a/packer/plugin/hook.go
+++ b/packer/plugin/hook.go
@@ -19,8 +19,17 @@ func (c *cmdHook) Run(name string, ui packer.Ui, comm packer.Communicator, data
 	return c.hook.Run(name, ui, comm, data)
 }
 
+func (c *cmdHook) Cancel() {
+	defer func() {
+		r := recover()
+		c.checkExit(r, nil)
+	}()
+
+	c.hook.Cancel()
+}
+
 func (c *cmdHook) checkExit(p interface{}, cb func()) {
-	if c.client.Exited() {
+	if c.client.Exited() && cb != nil {
 		cb()
 	} else if p != nil && !Killed {
 		log.Panic(p)
diff --git a/packer/plugin/hook_test.go b/packer/plugin/hook_test.go
index e6880e616..6f897da19 100644
--- a/packer/plugin/hook_test.go
+++ b/packer/plugin/hook_test.go
@@ -1,17 +1,10 @@
 package plugin
 
 import (
-	"github.com/mitchellh/packer/packer"
 	"os/exec"
 	"testing"
 )
 
-type helperHook byte
-
-func (helperHook) Run(string, packer.Ui, packer.Communicator, interface{}) error {
-	return nil
-}
-
 func TestHook_NoExist(t *testing.T) {
 	c := NewClient(&ClientConfig{Cmd: exec.Command("i-should-not-exist")})
 	defer c.Kill()
diff --git a/packer/plugin/plugin_test.go b/packer/plugin/plugin_test.go
index 0610887ec..7adaec5da 100644
--- a/packer/plugin/plugin_test.go
+++ b/packer/plugin/plugin_test.go
@@ -2,6 +2,7 @@ package plugin
 
 import (
 	"fmt"
+	"github.com/mitchellh/packer/packer"
 	"log"
 	"os"
 	"os/exec"
@@ -54,7 +55,7 @@ func TestHelperProcess(*testing.T) {
 	case "command":
 		ServeCommand(new(helperCommand))
 	case "hook":
-		ServeHook(new(helperHook))
+		ServeHook(new(packer.MockHook))
 	case "invalid-rpc-address":
 		fmt.Println("lolinvalid")
 	case "mock":
diff --git a/packer/provisioner.go b/packer/provisioner.go
index 99a0a2ac2..bee9a0961 100644
--- a/packer/provisioner.go
+++ b/packer/provisioner.go
@@ -32,3 +32,8 @@ func (h *ProvisionHook) Run(name string, ui Ui, comm Communicator, data interfac
 
 	return nil
 }
+
+// Cancels the privisioners that are still running.
+func (h *ProvisionHook) Cancel() {
+	// TODO(mitchellh): implement
+}
diff --git a/packer/rpc/builder_test.go b/packer/rpc/builder_test.go
index 8dff67b30..5235de9b5 100644
--- a/packer/rpc/builder_test.go
+++ b/packer/rpc/builder_test.go
@@ -72,7 +72,7 @@ func TestBuilderRPC(t *testing.T) {
 
 	// Test Run
 	cache := new(testCache)
-	hook := &testHook{}
+	hook := &packer.MockHook{}
 	ui := &testUi{}
 	artifact, err := bClient.Run(ui, hook, cache)
 	assert.Nil(err, "should have no error")
@@ -83,7 +83,7 @@ func TestBuilderRPC(t *testing.T) {
 		assert.True(cache.lockCalled, "lock should be called")
 
 		b.runHook.Run("foo", nil, nil, nil)
-		assert.True(hook.runCalled, "run should be called")
+		assert.True(hook.RunCalled, "run should be called")
 
 		b.runUi.Say("format")
 		assert.True(ui.sayCalled, "say should be called")
diff --git a/packer/rpc/hook.go b/packer/rpc/hook.go
index 27840f080..ea8fb6eef 100644
--- a/packer/rpc/hook.go
+++ b/packer/rpc/hook.go
@@ -37,6 +37,10 @@ func (h *hook) Run(name string, ui packer.Ui, comm packer.Communicator, data int
 	return h.client.Call("Hook.Run", args, new(interface{}))
 }
 
+func (h *hook) Cancel() {
+	// TODO(mitchellh): implement
+}
+
 func (h *HookServer) Run(args *HookRunArgs, reply *interface{}) error {
 	client, err := rpc.Dial("tcp", args.RPCAddress)
 	if err != nil {
diff --git a/packer/rpc/hook_test.go b/packer/rpc/hook_test.go
index fcc2aea7a..ea780a74e 100644
--- a/packer/rpc/hook_test.go
+++ b/packer/rpc/hook_test.go
@@ -7,21 +7,11 @@ import (
 	"testing"
 )
 
-type testHook struct {
-	runCalled bool
-	runUi     packer.Ui
-}
-
-func (h *testHook) Run(name string, ui packer.Ui, comm packer.Communicator, data interface{}) error {
-	h.runCalled = true
-	return nil
-}
-
 func TestHookRPC(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
 	// Create the UI to test
-	h := new(testHook)
+	h := new(packer.MockHook)
 
 	// Serve
 	server := rpc.NewServer()
@@ -37,7 +27,7 @@ func TestHookRPC(t *testing.T) {
 	// Test Run
 	ui := &testUi{}
 	hClient.Run("foo", ui, nil, 42)
-	assert.True(h.runCalled, "run should be called")
+	assert.True(h.RunCalled, "run should be called")
 }
 
 func TestHook_Implements(t *testing.T) {