diff --git a/packer/build.go b/packer/build.go
index 8d6cb1a60..8f93d1b31 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -20,7 +20,7 @@ type Build struct {
 // Run is where the actual build should take place. It takes a Build and a Ui.
 type Builder interface {
 	Prepare(config interface{})
-	Run(build Build, ui Ui)
+	Run(build *Build, ui Ui)
 }
 
 // This factory is responsible for returning Builders for the given name.
@@ -38,3 +38,14 @@ type NilBuilderFactory byte
 func (NilBuilderFactory) CreateBuilder(name string) Builder {
 	return nil
 }
+
+// Prepare prepares the build by doing some initialization for the builder
+// and any hooks. This _must_ be called prior to Run.
+func (b *Build) Prepare(config interface{}) {
+	b.builder.Prepare(config)
+}
+
+// Runs the actual build. Prepare must be called prior to running this.
+func (b *Build) Run(ui Ui) {
+	b.builder.Run(b, ui)
+}
diff --git a/packer/build_test.go b/packer/build_test.go
new file mode 100644
index 000000000..cfa05f7ac
--- /dev/null
+++ b/packer/build_test.go
@@ -0,0 +1,59 @@
+package packer
+
+import (
+	"cgl.tideland.biz/asserts"
+	"testing"
+)
+
+type TestBuilder struct {
+	prepareCalled bool
+	prepareConfig interface{}
+	runCalled bool
+	runBuild *Build
+	runUi Ui
+}
+
+func (tb *TestBuilder) Prepare(config interface{}) {
+	tb.prepareCalled = true
+	tb.prepareConfig = config
+}
+
+func (tb *TestBuilder) Run(b *Build, ui Ui) {
+	tb.runCalled = true
+	tb.runBuild = b
+	tb.runUi = ui
+}
+
+func testBuild() *Build {
+	return &Build{
+		name: "test",
+		builder: &TestBuilder{},
+	}
+}
+
+func TestBuild_Prepare(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	build := testBuild()
+	build.Prepare(42)
+
+	builder := build.builder.(*TestBuilder)
+
+	assert.True(builder.prepareCalled, "prepare should be called")
+	assert.Equal(builder.prepareConfig, 42, "prepare config should be 42")
+}
+
+func TestBuild_Run(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	ui := testUi()
+
+	build := testBuild()
+	build.Run(ui)
+
+	builder := build.builder.(*TestBuilder)
+
+	assert.True(builder.runCalled, "run should be called")
+	assert.Equal(builder.runBuild, build, "run should be called with build")
+	assert.Equal(builder.runUi, ui, "run should be called with ui")
+}
diff --git a/packer/ui_test.go b/packer/ui_test.go
index ccc0460d3..2fa0fa9e1 100644
--- a/packer/ui_test.go
+++ b/packer/ui_test.go
@@ -6,12 +6,18 @@ import (
 	"testing"
 )
 
-// Our test Ui that just writes to bytes.Buffers.
-var bufferUi = &ReaderWriterUi{new(bytes.Buffer), new(bytes.Buffer)}
+func testUi() *ReaderWriterUi {
+	return &ReaderWriterUi{
+		new(bytes.Buffer),
+		new(bytes.Buffer),
+	}
+}
 
 func TestReaderWriterUi_Say(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
+	bufferUi := testUi()
+
 	bufferUi.Say("foo")
 	assert.Equal(readWriter(bufferUi), "foo", "basic output")