diff --git a/packer/build.go b/packer/build.go
index b4d30fa56..bff34767e 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -23,6 +23,10 @@ const (
 	// This is the key in configurations that is set to "true" when Packer
 	// force build is enabled.
 	ForceConfigKey = "packer_force"
+
+	// This key contains a map[string]string of the user variables for
+	// template processing.
+	UserVariablesConfigKey = "packer_user_variables"
 )
 
 // A Build represents a single job within Packer that is responsible for
@@ -105,7 +109,7 @@ 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. The parameter is the
 // overrides for the variables within the template (if any).
-func (b *coreBuild) Prepare(v map[string]string) (err error) {
+func (b *coreBuild) Prepare(userVars map[string]string) (err error) {
 	b.l.Lock()
 	defer b.l.Unlock()
 
@@ -115,11 +119,37 @@ func (b *coreBuild) Prepare(v map[string]string) (err error) {
 
 	b.prepareCalled = true
 
+	// Compile the variables
+	variables := make(map[string]string)
+	for k, v := range b.variables {
+		variables[k] = v
+	}
+
+	if userVars != nil {
+		errs := make([]error, 0)
+		for k, v := range userVars {
+			if _, ok := variables[k]; !ok {
+				errs = append(
+					errs, fmt.Errorf("Unknown user variable: %s", k))
+				continue
+			}
+
+			variables[k] = v
+		}
+
+		if len(errs) > 0 {
+			return &MultiError{
+				Errors: errs,
+			}
+		}
+	}
+
 	packerConfig := map[string]interface{}{
-		BuildNameConfigKey:   b.name,
-		BuilderTypeConfigKey: b.builderType,
-		DebugConfigKey:       b.debug,
-		ForceConfigKey:       b.force,
+		BuildNameConfigKey:     b.name,
+		BuilderTypeConfigKey:   b.builderType,
+		DebugConfigKey:         b.debug,
+		ForceConfigKey:         b.force,
+		UserVariablesConfigKey: variables,
 	}
 
 	// Prepare the builder
diff --git a/packer/build_test.go b/packer/build_test.go
index 00b77e6be..baf33cfa9 100644
--- a/packer/build_test.go
+++ b/packer/build_test.go
@@ -23,6 +23,7 @@ func testBuild() *coreBuild {
 				coreBuildPostProcessor{&TestPostProcessor{artifactId: "pp"}, "testPP", 42, true},
 			},
 		},
+		variables: make(map[string]string),
 	}
 }
 
@@ -30,6 +31,15 @@ func testBuilder() *TestBuilder {
 	return &TestBuilder{}
 }
 
+func testDefaultPackerConfig() map[string]interface{} {
+	return map[string]interface{}{
+		BuildNameConfigKey:     "test",
+		BuilderTypeConfigKey:   "foo",
+		DebugConfigKey:         false,
+		ForceConfigKey:         false,
+		UserVariablesConfigKey: make(map[string]string),
+	}
+}
 func TestBuild_Name(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
@@ -39,13 +49,7 @@ func TestBuild_Name(t *testing.T) {
 
 func TestBuild_Prepare(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
-
-	packerConfig := map[string]interface{}{
-		BuildNameConfigKey:   "test",
-		BuilderTypeConfigKey: "foo",
-		DebugConfigKey:       false,
-		ForceConfigKey:       false,
-	}
+	packerConfig := testDefaultPackerConfig()
 
 	build := testBuild()
 	builder := build.builder.(*TestBuilder)
@@ -88,12 +92,8 @@ func TestBuild_Prepare_Twice(t *testing.T) {
 func TestBuild_Prepare_Debug(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
-	packerConfig := map[string]interface{}{
-		BuildNameConfigKey:   "test",
-		BuilderTypeConfigKey: "foo",
-		DebugConfigKey:       true,
-		ForceConfigKey:       false,
-	}
+	packerConfig := testDefaultPackerConfig()
+	packerConfig[DebugConfigKey] = true
 
 	build := testBuild()
 	builder := build.builder.(*TestBuilder)
@@ -109,6 +109,64 @@ func TestBuild_Prepare_Debug(t *testing.T) {
 	assert.Equal(prov.prepConfigs, []interface{}{42, packerConfig}, "prepare should be called with proper config")
 }
 
+func TestBuildPrepare_variables_default(t *testing.T) {
+	packerConfig := testDefaultPackerConfig()
+	packerConfig[UserVariablesConfigKey] = map[string]string{
+		"foo": "bar",
+	}
+
+	build := testBuild()
+	build.variables["foo"] = "bar"
+	builder := build.builder.(*TestBuilder)
+
+	err := build.Prepare(nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if !builder.prepareCalled {
+		t.Fatal("prepare should be called")
+	}
+
+	if !reflect.DeepEqual(builder.prepareConfig[1], packerConfig) {
+		t.Fatalf("prepare bad: %#v", builder.prepareConfig[1])
+	}
+}
+
+func TestBuildPrepare_variables_nonexist(t *testing.T) {
+	build := testBuild()
+	build.variables["foo"] = "bar"
+
+	err := build.Prepare(map[string]string{"bar": "baz"})
+	if err == nil {
+		t.Fatal("should have had error")
+	}
+}
+
+func TestBuildPrepare_variables_override(t *testing.T) {
+	packerConfig := testDefaultPackerConfig()
+	packerConfig[UserVariablesConfigKey] = map[string]string{
+		"foo": "baz",
+	}
+
+	build := testBuild()
+	build.variables["foo"] = "bar"
+	builder := build.builder.(*TestBuilder)
+
+	err := build.Prepare(map[string]string{"foo": "baz"})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if !builder.prepareCalled {
+		t.Fatal("prepare should be called")
+	}
+
+	if !reflect.DeepEqual(builder.prepareConfig[1], packerConfig) {
+		t.Fatalf("prepare bad: %#v", builder.prepareConfig[1])
+	}
+}
+
 func TestBuild_Run(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)