diff --git a/packer/build.go b/packer/build.go
index f88044a6c..b4d30fa56 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -35,7 +35,7 @@ type Build interface {
 
 	// Prepare configures the various components of this build and reports
 	// any errors in doing so (such as syntax errors, validation errors, etc.)
-	Prepare() error
+	Prepare(v map[string]string) error
 
 	// Run runs the actual builder, returning an artifact implementation
 	// of what is built. If anything goes wrong, an error is returned.
@@ -103,8 +103,9 @@ func (b *coreBuild) Name() string {
 }
 
 // Prepare prepares the build by doing some initialization for the builder
-// and any hooks. This _must_ be called prior to Run.
-func (b *coreBuild) Prepare() (err error) {
+// and any hooks. This _must_ be called prior to Run. The parameter is the
+// overrides for the variables within the template (if any).
+func (b *coreBuild) Prepare(v map[string]string) (err error) {
 	b.l.Lock()
 	defer b.l.Unlock()
 
diff --git a/packer/build_test.go b/packer/build_test.go
index ac4df90b5..00b77e6be 100644
--- a/packer/build_test.go
+++ b/packer/build_test.go
@@ -50,7 +50,7 @@ func TestBuild_Prepare(t *testing.T) {
 	build := testBuild()
 	builder := build.builder.(*TestBuilder)
 
-	build.Prepare()
+	build.Prepare(nil)
 	assert.True(builder.prepareCalled, "prepare should be called")
 	assert.Equal(builder.prepareConfig, []interface{}{42, packerConfig}, "prepare config should be 42")
 
@@ -67,7 +67,7 @@ func TestBuild_Prepare(t *testing.T) {
 
 func TestBuild_Prepare_Twice(t *testing.T) {
 	build := testBuild()
-	if err := build.Prepare(); err != nil {
+	if err := build.Prepare(nil); err != nil {
 		t.Fatalf("bad error: %s", err)
 	}
 
@@ -82,7 +82,7 @@ func TestBuild_Prepare_Twice(t *testing.T) {
 		}
 	}()
 
-	build.Prepare()
+	build.Prepare(nil)
 }
 
 func TestBuild_Prepare_Debug(t *testing.T) {
@@ -99,7 +99,7 @@ func TestBuild_Prepare_Debug(t *testing.T) {
 	builder := build.builder.(*TestBuilder)
 
 	build.SetDebug(true)
-	build.Prepare()
+	build.Prepare(nil)
 	assert.True(builder.prepareCalled, "prepare should be called")
 	assert.Equal(builder.prepareConfig, []interface{}{42, packerConfig}, "prepare config should be 42")
 
@@ -116,7 +116,7 @@ func TestBuild_Run(t *testing.T) {
 	ui := testUi()
 
 	build := testBuild()
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err := build.Run(ui, cache)
 	assert.Nil(err, "should not error")
 	assert.Equal(len(artifacts), 2, "should have two artifacts")
@@ -152,7 +152,7 @@ func TestBuild_Run_Artifacts(t *testing.T) {
 	build := testBuild()
 	build.postProcessors = [][]coreBuildPostProcessor{}
 
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err := build.Run(ui, cache)
 	if err != nil {
 		t.Fatalf("err: %s", err)
@@ -177,7 +177,7 @@ func TestBuild_Run_Artifacts(t *testing.T) {
 		},
 	}
 
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err = build.Run(ui, cache)
 	if err != nil {
 		t.Fatalf("err: %s", err)
@@ -205,7 +205,7 @@ func TestBuild_Run_Artifacts(t *testing.T) {
 		},
 	}
 
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err = build.Run(ui, cache)
 	if err != nil {
 		t.Fatalf("err: %s", err)
@@ -235,7 +235,7 @@ func TestBuild_Run_Artifacts(t *testing.T) {
 		},
 	}
 
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err = build.Run(ui, cache)
 	if err != nil {
 		t.Fatalf("err: %s", err)
@@ -262,7 +262,7 @@ func TestBuild_Run_Artifacts(t *testing.T) {
 		},
 	}
 
-	build.Prepare()
+	build.Prepare(nil)
 	artifacts, err = build.Run(ui, cache)
 	if err != nil {
 		t.Fatalf("err: %s", err)
diff --git a/packer/rpc/build.go b/packer/rpc/build.go
index 537566bfe..6d81ce870 100644
--- a/packer/rpc/build.go
+++ b/packer/rpc/build.go
@@ -30,8 +30,8 @@ func (b *build) Name() (result string) {
 	return
 }
 
-func (b *build) Prepare() (err error) {
-	if cerr := b.client.Call("Build.Prepare", new(interface{}), &err); cerr != nil {
+func (b *build) Prepare(v map[string]string) (err error) {
+	if cerr := b.client.Call("Build.Prepare", v, &err); cerr != nil {
 		return cerr
 	}
 
@@ -86,8 +86,8 @@ func (b *BuildServer) Name(args *interface{}, reply *string) error {
 	return nil
 }
 
-func (b *BuildServer) Prepare(args interface{}, reply *error) error {
-	*reply = b.build.Prepare()
+func (b *BuildServer) Prepare(v map[string]string, reply *error) error {
+	*reply = b.build.Prepare(v)
 	return nil
 }
 
diff --git a/packer/rpc/build_test.go b/packer/rpc/build_test.go
index 1d420dbbb..e8555ffff 100644
--- a/packer/rpc/build_test.go
+++ b/packer/rpc/build_test.go
@@ -13,6 +13,7 @@ var testBuildArtifact = &testArtifact{}
 type testBuild struct {
 	nameCalled     bool
 	prepareCalled  bool
+	prepareVars    map[string]string
 	runCalled      bool
 	runCache       packer.Cache
 	runUi          packer.Ui
@@ -28,8 +29,9 @@ func (b *testBuild) Name() string {
 	return "name"
 }
 
-func (b *testBuild) Prepare() error {
+func (b *testBuild) Prepare(v map[string]string) error {
 	b.prepareCalled = true
+	b.prepareVars = v
 	return nil
 }
 
@@ -78,8 +80,15 @@ func TestBuildRPC(t *testing.T) {
 	assert.True(b.nameCalled, "name should be called")
 
 	// Test Prepare
-	bClient.Prepare()
+	bClient.Prepare(map[string]string{"foo": "bar"})
 	assert.True(b.prepareCalled, "prepare should be called")
+	if len(b.prepareVars) != 1 {
+		t.Fatalf("bad vars: %#v", b.prepareVars)
+	}
+
+	if b.prepareVars["foo"] != "bar" {
+		t.Fatalf("bad vars: %#v", b.prepareVars)
+	}
 
 	// Test Run
 	cache := new(testCache)