diff --git a/terraform/terraform.go b/terraform/terraform.go
index 82238b2a6..468d2866e 100644
--- a/terraform/terraform.go
+++ b/terraform/terraform.go
@@ -21,11 +21,10 @@ type Terraform struct {
 // terraformProvider contains internal state information about a resource
 // provider for Terraform.
 type terraformProvider struct {
-	Provider   ResourceProvider
-	Config     *config.ProviderConfig
-	Configured bool
+	Provider ResourceProvider
+	Config   *config.ProviderConfig
 
-	sync.Mutex
+	sync.Once
 }
 
 // Config is the configuration that must be given to instantiate
@@ -134,7 +133,8 @@ func (t *Terraform) diffWalkFn(
 			panic(fmt.Sprintf("No provider for resource: %s", r.Id()))
 		}
 
-		// TODO(mitchellh): initialize provider if we haven't
+		// Initialize the provider if we haven't already
+		p.init(vars)
 
 		l.RLock()
 		var rs *ResourceState
@@ -172,3 +172,18 @@ func (t *Terraform) diffWalkFn(
 		return nil
 	}
 }
+
+func (t *terraformProvider) init(vars map[string]string) error {
+	var err error
+
+	t.Once.Do(func() {
+		var c map[string]interface{}
+		if t.Config != nil {
+			c = t.Config.ReplaceVariables(vars).Config
+		}
+
+		_, err = t.Provider.Configure(c)
+	})
+
+	return err
+}
diff --git a/terraform/terraform_test.go b/terraform/terraform_test.go
index 44173cfdf..c429292e9 100644
--- a/terraform/terraform_test.go
+++ b/terraform/terraform_test.go
@@ -221,6 +221,26 @@ func TestTerraformDiff_computed(t *testing.T) {
 	}
 }
 
+func TestTerraformDiff_providerInit(t *testing.T) {
+	tf := testTerraform(t, "diff-provider-init")
+
+	_, err := tf.Diff(nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	p := testProviderMock(testProvider(tf, "do_droplet.bar"))
+	if p == nil {
+		t.Fatal("should have provider")
+	}
+	if !p.ConfigureCalled {
+		t.Fatal("configure should be called")
+	}
+	if p.ConfigureConfig["foo"].(string) != "2" {
+		t.Fatalf("bad: %#v", p.ConfigureConfig)
+	}
+}
+
 func testConfig(t *testing.T, name string) *config.Config {
 	c, err := config.Load(filepath.Join(fixtureDir, name, "main.tf"))
 	if err != nil {
diff --git a/terraform/test-fixtures/diff-provider-init/main.tf b/terraform/test-fixtures/diff-provider-init/main.tf
new file mode 100644
index 000000000..baa619eb7
--- /dev/null
+++ b/terraform/test-fixtures/diff-provider-init/main.tf
@@ -0,0 +1,9 @@
+provider "do" {
+  foo = "${aws_instance.foo.num}"
+}
+
+resource "aws_instance" "foo" {
+    num = "2"
+}
+
+resource "do_droplet" "bar" {}