diff --git a/terraform/resource.go b/terraform/resource.go
index a9db7e6a4..202116d63 100644
--- a/terraform/resource.go
+++ b/terraform/resource.go
@@ -15,7 +15,8 @@ type Resource struct {
 	State    *ResourceState
 }
 
-// TODO: test
+// Vars returns the mapping of variables that should be replaced in
+// configuration based on the attributes of this resource.
 func (r *Resource) Vars() map[string]string {
 	if r.State == nil {
 		return nil
diff --git a/terraform/resource_test.go b/terraform/resource_test.go
new file mode 100644
index 000000000..9837cee29
--- /dev/null
+++ b/terraform/resource_test.go
@@ -0,0 +1,31 @@
+package terraform
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestResource_Vars(t *testing.T) {
+	r := new(Resource)
+
+	if len(r.Vars()) > 0 {
+		t.Fatalf("bad: %#v", r.Vars())
+	}
+
+	r = &Resource{
+		Id: "key",
+		State: &ResourceState{
+			Attributes: map[string]string{
+				"foo": "bar",
+			},
+		},
+	}
+
+	expected := map[string]string{
+		"key.foo": "bar",
+	}
+	actual := r.Vars()
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad: %#v", actual)
+	}
+}