diff --git a/Makefile b/Makefile
index 028fd3feb1..012e4a9ce4 100644
--- a/Makefile
+++ b/Makefile
@@ -19,7 +19,7 @@ default: test
 libucl: vendor/libucl/$(LIBUCL_NAME)
 
 test: libucl
-	go test $(TEST)
+	go test $(TEST) -timeout=5s
 
 vendor/libucl/libucl.a: vendor/libucl
 	cd vendor/libucl && \
diff --git a/terraform/diff.go b/terraform/diff.go
index 725800d2f6..9e54ece7bf 100644
--- a/terraform/diff.go
+++ b/terraform/diff.go
@@ -1,15 +1,26 @@
 package terraform
 
+import (
+	"sync"
+)
+
 // Diff tracks the differences between resources to apply.
 type Diff struct {
-	resources map[string]map[string]*resourceDiff
+	Resources map[string]map[string]*ResourceAttrDiff
+	once      sync.Once
+}
+
+func (d *Diff) init() {
+	d.once.Do(func() {
+		d.Resources = make(map[string]map[string]*ResourceAttrDiff)
+	})
 }
 
-// resourceDiff is the diff of a single attribute of a resource.
+// ResourceAttrDiff is the diff of a single attribute of a resource.
 //
 // This tracks the old value, the new value, and whether the change of this
 // value actually requires an entirely new resource.
-type resourceDiff struct {
+type ResourceAttrDiff struct {
 	Old         string
 	New         string
 	RequiresNew bool
diff --git a/terraform/resource_provider.go b/terraform/resource_provider.go
index 67100de259..e4cccedcaa 100644
--- a/terraform/resource_provider.go
+++ b/terraform/resource_provider.go
@@ -30,14 +30,7 @@ type ResourceProvider interface {
 
 // ResourceDiff is the diff of a resource from some state to another.
 type ResourceDiff struct {
-	Attributes map[string]ResourceDiffAttribute
-}
-
-// ResourceDiffAttribute is the diff of a single attribute of a resource.
-type ResourceDiffAttribute struct {
-	Old         string
-	New         string
-	RequiresNew bool
+	Attributes map[string]*ResourceAttrDiff
 }
 
 // ResourceState holds the state of a resource that is used so that
diff --git a/terraform/resource_provider_mock.go b/terraform/resource_provider_mock.go
index f648ca9550..b3e62ae567 100644
--- a/terraform/resource_provider_mock.go
+++ b/terraform/resource_provider_mock.go
@@ -10,11 +10,11 @@ type MockResourceProvider struct {
 	ConfigureConfig         map[string]interface{}
 	ConfigureReturnWarnings []string
 	ConfigureReturnError    error
-	ResourceDiffCalled      bool
-	ResourceDiffState       ResourceState
-	ResourceDiffDesired     map[string]interface{}
-	ResourceDiffReturn      ResourceDiff
-	ResourceDiffReturnError error
+	DiffCalled              bool
+	DiffState               ResourceState
+	DiffDesired             map[string]interface{}
+	DiffReturn              ResourceDiff
+	DiffReturnError         error
 	ResourcesCalled         bool
 	ResourcesReturn         []ResourceType
 }
@@ -28,10 +28,10 @@ func (p *MockResourceProvider) Configure(c map[string]interface{}) ([]string, er
 func (p *MockResourceProvider) Diff(
 	state ResourceState,
 	desired map[string]interface{}) (ResourceDiff, error) {
-	p.ResourceDiffCalled = true
-	p.ResourceDiffState = state
-	p.ResourceDiffDesired = desired
-	return p.ResourceDiffReturn, p.ResourceDiffReturnError
+	p.DiffCalled = true
+	p.DiffState = state
+	p.DiffDesired = desired
+	return p.DiffReturn, p.DiffReturnError
 }
 
 func (p *MockResourceProvider) Resources() []ResourceType {
diff --git a/terraform/terraform.go b/terraform/terraform.go
index 3634a133f3..d8201687ac 100644
--- a/terraform/terraform.go
+++ b/terraform/terraform.go
@@ -3,6 +3,7 @@ package terraform
 import (
 	"fmt"
 	"strings"
+	"sync"
 
 	"github.com/hashicorp/terraform/config"
 	"github.com/hashicorp/terraform/depgraph"
@@ -124,14 +125,57 @@ func (t *Terraform) Apply(*State, *Diff) (*State, error) {
 	return nil, nil
 }
 
-func (t *Terraform) Diff(*State) (*Diff, error) {
-	return nil, nil
+func (t *Terraform) Diff(s *State) (*Diff, error) {
+	result := new(Diff)
+	err := t.graph.Walk(t.diffWalkFn(s, result))
+	if err != nil {
+		return nil, err
+	}
+
+	return result, nil
 }
 
 func (t *Terraform) Refresh(*State) (*State, error) {
 	return nil, nil
 }
 
+func (t *Terraform) diffWalkFn(
+	state *State, result *Diff) depgraph.WalkFunc {
+	var resultLock sync.Mutex
+
+	return func(n *depgraph.Noun) error {
+		// If it is the root node, ignore
+		if n.Name == config.ResourceGraphRoot {
+			return nil
+		}
+
+		r := n.Meta.(*config.Resource)
+		p := t.mapping[r]
+		if p == nil {
+			panic(fmt.Sprintf("No provider for resource: %s", r.Id()))
+		}
+
+		var rs ResourceState
+		diff, err := p.Diff(rs, r.Config)
+		if err != nil {
+			return err
+		}
+
+		// If there were no diff items, return right away
+		if len(diff.Attributes) == 0 {
+			return nil
+		}
+
+		// Acquire a lock and modify the resulting diff
+		resultLock.Lock()
+		defer resultLock.Unlock()
+		result.init()
+		result.Resources[r.Id()] = diff.Attributes
+
+		return nil
+	}
+}
+
 // matchingPrefixes takes a resource type and a set of resource
 // providers we know about by prefix and returns a list of prefixes
 // that might be valid for that resource.
diff --git a/terraform/terraform_test.go b/terraform/terraform_test.go
index 1821257d30..a918a585bb 100644
--- a/terraform/terraform_test.go
+++ b/terraform/terraform_test.go
@@ -10,46 +10,6 @@ import (
 // This is the directory where our test fixtures are.
 const fixtureDir = "./test-fixtures"
 
-func testConfig(t *testing.T, name string) *config.Config {
-	c, err := config.Load(filepath.Join(fixtureDir, name, "main.tf"))
-	if err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	return c
-}
-
-func testProviderFunc(n string, rs []string) ResourceProviderFactory {
-	resources := make([]ResourceType, len(rs))
-	for i, v := range rs {
-		resources[i] = ResourceType{
-			Name: v,
-		}
-	}
-
-	return func() (ResourceProvider, error) {
-		result := &MockResourceProvider{
-			Meta:            n,
-			ResourcesReturn: resources,
-		}
-
-		return result, nil
-	}
-}
-
-func testProviderName(p ResourceProvider) string {
-	return p.(*MockResourceProvider).Meta.(string)
-}
-
-func testResourceMapping(tf *Terraform) map[string]ResourceProvider {
-	result := make(map[string]ResourceProvider)
-	for resource, provider := range tf.mapping {
-		result[resource.Id()] = provider
-	}
-
-	return result
-}
-
 func TestNew(t *testing.T) {
 	config := testConfig(t, "new-good")
 	tfConfig := &Config{
@@ -141,3 +101,86 @@ func TestNew_variables(t *testing.T) {
 		t.Fatal("tf should not be nil")
 	}
 }
+
+func TestTerraformDiff(t *testing.T) {
+	tf := testTerraform(t, "diff-good")
+
+	diff, err := tf.Diff(nil)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if len(diff.Resources) < 2 {
+		t.Fatalf("bad: %#v", diff.Resources)
+	}
+}
+
+func testConfig(t *testing.T, name string) *config.Config {
+	c, err := config.Load(filepath.Join(fixtureDir, name, "main.tf"))
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	return c
+}
+
+func testProviderFunc(n string, rs []string) ResourceProviderFactory {
+	resources := make([]ResourceType, len(rs))
+	for i, v := range rs {
+		resources[i] = ResourceType{
+			Name: v,
+		}
+	}
+
+	return func() (ResourceProvider, error) {
+		var diff ResourceDiff
+		diff.Attributes = map[string]*ResourceAttrDiff{
+			n: &ResourceAttrDiff{
+				Old: "foo",
+				New: "bar",
+			},
+		}
+
+		result := &MockResourceProvider{
+			Meta:            n,
+			DiffReturn:      diff,
+			ResourcesReturn: resources,
+		}
+
+		return result, nil
+	}
+}
+
+func testProviderName(p ResourceProvider) string {
+	return p.(*MockResourceProvider).Meta.(string)
+}
+
+func testResourceMapping(tf *Terraform) map[string]ResourceProvider {
+	result := make(map[string]ResourceProvider)
+	for resource, provider := range tf.mapping {
+		result[resource.Id()] = provider
+	}
+
+	return result
+}
+
+func testTerraform(t *testing.T, name string) *Terraform {
+	config := testConfig(t, name)
+	tfConfig := &Config{
+		Config: config,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFunc("aws", []string{"aws_instance"}),
+			"do":  testProviderFunc("do", []string{"do_droplet"}),
+		},
+	}
+
+	tf, err := New(tfConfig)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if tf == nil {
+		t.Fatal("tf should not be nil")
+	}
+
+	return tf
+}
diff --git a/terraform/test-fixtures/diff-good/main.tf b/terraform/test-fixtures/diff-good/main.tf
new file mode 100644
index 0000000000..642251b951
--- /dev/null
+++ b/terraform/test-fixtures/diff-good/main.tf
@@ -0,0 +1,7 @@
+resource "aws_instance" "foo" {
+    num = 2
+}
+
+resource "aws_instance" "bar" {
+    foo = "${aws_instance.foo.num}"
+}