diff --git a/packer/build.go b/packer/build.go
index c707be974..2e3ddf2e5 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -9,6 +9,25 @@ type Build struct {
 	builder Builder
 }
 
+// Implementers of Builder are responsible for actually building images
+// on some platform given some configuration.
+//
+// Prepare is responsible for reading in some configuration, in the raw form
+// of map[string]interface{}, and storing that state for use later. Any setup
+// should be done in this method. Note that NO side effects should really take
+// place in prepare. It is meant as a state setup step only.
+//
+// Run is where the actual build should take place. It takes a Build and a Ui.
 type Builder interface {
-	Prepare()
+	Prepare(config interface{})
+	Run(build Build, ui Ui)
+}
+
+// This factory is responsible for returning Builders for the given name.
+//
+// CreateBuilder is called with the string name of a builder and should
+// return a Builder implementation by that name or nil if no builder can be
+// found.
+type BuilderFactory interface {
+	CreateBuilder(name string) Builder
 }