diff --git a/terraform/shadow_resource_provider.go b/terraform/shadow_resource_provider.go
index 63df15527..d3b74b29e 100644
--- a/terraform/shadow_resource_provider.go
+++ b/terraform/shadow_resource_provider.go
@@ -123,6 +123,21 @@ func (p *shadowResourceProviderReal) Apply(
 	return result, err
 }
 
+func (p *shadowResourceProviderReal) Diff(
+	info *InstanceInfo,
+	state *InstanceState,
+	desired *ResourceConfig) (*InstanceDiff, error) {
+	result, err := p.ResourceProvider.Diff(info, state, desired)
+	p.Shared.Diff.SetValue(info.HumanId(), &shadowResourceProviderDiff{
+		State:     state,
+		Desired:   desired,
+		Result:    result,
+		ResultErr: err,
+	})
+
+	return result, err
+}
+
 // shadowResourceProviderShadow is the shadow resource provider. Function
 // calls never affect real resources. This is paired with the "real" side
 // which must be called properly to enable recording.
@@ -143,6 +158,7 @@ type shadowResourceProviderShared struct {
 	Validate  shadow.Value
 	Configure shadow.Value
 	Apply     shadow.KeyedValue
+	Diff      shadow.KeyedValue
 }
 
 func (p *shadowResourceProviderShadow) CloseShadow() error {
@@ -296,6 +312,50 @@ func (p *shadowResourceProviderShadow) Apply(
 	return result.Result, result.ResultErr
 }
 
+func (p *shadowResourceProviderShadow) Diff(
+	info *InstanceInfo,
+	state *InstanceState,
+	desired *ResourceConfig) (*InstanceDiff, error) {
+	// Unique key
+	key := info.HumanId()
+	raw := p.Shared.Diff.Value(key)
+	if raw == nil {
+		p.ErrorLock.Lock()
+		defer p.ErrorLock.Unlock()
+		p.Error = multierror.Append(p.Error, fmt.Errorf(
+			"Unknown 'diff' call for %q:\n\n%#v\n\n%#v",
+			key, state, desired))
+		return nil, nil
+	}
+
+	result, ok := raw.(*shadowResourceProviderDiff)
+	if !ok {
+		p.ErrorLock.Lock()
+		defer p.ErrorLock.Unlock()
+		p.Error = multierror.Append(p.Error, fmt.Errorf(
+			"Unknown 'diff' shadow value: %#v", raw))
+		return nil, nil
+	}
+
+	// Compare the parameters, which should be identical
+	if !state.Equal(result.State) {
+		p.ErrorLock.Lock()
+		p.Error = multierror.Append(p.Error, fmt.Errorf(
+			"Diff %q had unequal states (real, then shadow):\n\n%#v\n\n%#v",
+			key, result.State, state))
+		p.ErrorLock.Unlock()
+	}
+	if !desired.Equal(result.Desired) {
+		p.ErrorLock.Lock()
+		p.Error = multierror.Append(p.Error, fmt.Errorf(
+			"Diff %q had unequal states (real, then shadow):\n\n%#v\n\n%#v",
+			key, result.Desired, desired))
+		p.ErrorLock.Unlock()
+	}
+
+	return result.Result, result.ResultErr
+}
+
 // TODO
 // TODO
 // TODO
@@ -306,13 +366,6 @@ func (p *shadowResourceProviderShadow) ValidateResource(t string, c *ResourceCon
 	return nil, nil
 }
 
-func (p *shadowResourceProviderShadow) Diff(
-	info *InstanceInfo,
-	state *InstanceState,
-	desired *ResourceConfig) (*InstanceDiff, error) {
-	return nil, nil
-}
-
 func (p *shadowResourceProviderShadow) Refresh(
 	info *InstanceInfo,
 	s *InstanceState) (*InstanceState, error) {
@@ -365,3 +418,10 @@ type shadowResourceProviderApply struct {
 	Result    *InstanceState
 	ResultErr error
 }
+
+type shadowResourceProviderDiff struct {
+	State     *InstanceState
+	Desired   *ResourceConfig
+	Result    *InstanceDiff
+	ResultErr error
+}
diff --git a/terraform/shadow_resource_provider_test.go b/terraform/shadow_resource_provider_test.go
index 4c185f3ce..622a92631 100644
--- a/terraform/shadow_resource_provider_test.go
+++ b/terraform/shadow_resource_provider_test.go
@@ -319,3 +319,57 @@ func TestShadowResourceProviderApply(t *testing.T) {
 		t.Fatalf("bad: %s", err)
 	}
 }
+
+func TestShadowResourceProviderDiff(t *testing.T) {
+	mock := new(MockResourceProvider)
+	real, shadow := newShadowResourceProvider(mock)
+
+	// Test values
+	info := &InstanceInfo{Id: "foo"}
+	state := &InstanceState{ID: "foo"}
+	desired := testResourceConfig(t, map[string]interface{}{"foo": "bar"})
+	mockResult := &InstanceDiff{Destroy: true}
+
+	// Configure the mock
+	mock.DiffReturn = mockResult
+
+	// Verify that it blocks until the real func is called
+	var result *InstanceDiff
+	var err error
+	doneCh := make(chan struct{})
+	go func() {
+		defer close(doneCh)
+		result, err = shadow.Diff(info, state, desired)
+	}()
+
+	select {
+	case <-doneCh:
+		t.Fatal("should block until finished")
+	case <-time.After(10 * time.Millisecond):
+	}
+
+	// Call the real func
+	realResult, realErr := real.Diff(info, state, desired)
+	if !reflect.DeepEqual(realResult, mockResult) {
+		t.Fatalf("bad: %#v", realResult)
+	}
+	if realErr != nil {
+		t.Fatalf("bad: %#v", realErr)
+	}
+
+	// The shadow should finish now
+	<-doneCh
+
+	// Verify the shadow returned the same values
+	if !reflect.DeepEqual(result, mockResult) {
+		t.Fatalf("bad: %#v", result)
+	}
+	if err != nil {
+		t.Fatalf("bad: %#v", err)
+	}
+
+	// Verify we have no errors
+	if err := shadow.CloseShadow(); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+}