diff --git a/config/config.go b/config/config.go
index bb7ab76bf8..cab5f728a1 100644
--- a/config/config.go
+++ b/config/config.go
@@ -4,10 +4,11 @@ package config
 
 import (
 	"fmt"
-	"strconv"
 	"strings"
 
+	"github.com/hashicorp/terraform/flatmap"
 	"github.com/hashicorp/terraform/helper/multierror"
+	"github.com/mitchellh/mapstructure"
 )
 
 // Config is the configuration that comes from loading a collection
@@ -53,9 +54,8 @@ type Provisioner struct {
 // Variable is a variable defined within the configuration.
 type Variable struct {
 	Name        string
-	Default     string
+	Default     interface{}
 	Description string
-	defaultSet  bool
 }
 
 // Output is an output defined within the configuration. An output is
@@ -65,37 +65,15 @@ type Output struct {
 	RawConfig *RawConfig
 }
 
-// An InterpolatedVariable is a variable that is embedded within a string
-// in the configuration, such as "hello ${world}" (world in this case is
-// an interpolated variable).
-//
-// These variables can come from a variety of sources, represented by
-// implementations of this interface.
-type InterpolatedVariable interface {
-	FullKey() string
-}
+// VariableType is the type of value a variable is holding, and returned
+// by the Type() function on variables.
+type VariableType byte
 
-// A ResourceVariable is a variable that is referencing the field
-// of a resource, such as "${aws_instance.foo.ami}"
-type ResourceVariable struct {
-	Type  string // Resource type, i.e. "aws_instance"
-	Name  string // Resource name
-	Field string // Resource field
-
-	Multi bool // True if multi-variable: aws_instance.foo.*.id
-	Index int  // Index for multi-variable: aws_instance.foo.1.id == 1
-
-	key string
-}
-
-// A UserVariable is a variable that is referencing a user variable
-// that is inputted from outside the configuration. This looks like
-// "${var.foo}"
-type UserVariable struct {
-	Name string
-
-	key string
-}
+const (
+	VariableTypeUnknown VariableType = iota
+	VariableTypeString
+	VariableTypeMap
+)
 
 // ProviderConfigName returns the name of the provider configuration in
 // the given mapping that maps to the proper provider configuration
@@ -132,6 +110,14 @@ func (c *Config) Validate() error {
 		varMap[v.Name] = v
 	}
 
+	for _, v := range c.Variables {
+		if v.Type() == VariableTypeUnknown {
+			errs = append(errs, fmt.Errorf(
+				"Variable '%s': must be string or mapping",
+				v.Name))
+		}
+	}
+
 	// Check for references to user variables that do not actually
 	// exist and record those errors.
 	for source, vs := range vars {
@@ -227,8 +213,8 @@ func (c *Config) Validate() error {
 // are valid in the Validate step.
 func (c *Config) allVariables() map[string][]InterpolatedVariable {
 	result := make(map[string][]InterpolatedVariable)
-	for n, pc := range c.ProviderConfigs {
-		source := fmt.Sprintf("provider config '%s'", n)
+	for _, pc := range c.ProviderConfigs {
+		source := fmt.Sprintf("provider config '%s'", pc.Name)
 		for _, v := range pc.RawConfig.Variables {
 			result[source] = append(result[source], v)
 		}
@@ -302,6 +288,28 @@ func (r *Resource) mergerMerge(m merger) merger {
 	return &result
 }
 
+// DefaultsMap returns a map of default values for this variable.
+func (v *Variable) DefaultsMap() map[string]string {
+	if v.Default == nil {
+		return nil
+	}
+
+	n := fmt.Sprintf("var.%s", v.Name)
+	switch v.Type() {
+	case VariableTypeString:
+		return map[string]string{n: v.Default.(string)}
+	case VariableTypeMap:
+		result := flatmap.Flatten(map[string]interface{}{
+			n: v.Default.(map[string]string),
+		})
+		result[n] = v.Name
+
+		return result
+	default:
+		return nil
+	}
+}
+
 // Merge merges two variables to create a new third variable.
 func (v *Variable) Merge(v2 *Variable) *Variable {
 	// Shallow copy the variable
@@ -310,9 +318,8 @@ func (v *Variable) Merge(v2 *Variable) *Variable {
 	// The names should be the same, but the second name always wins.
 	result.Name = v2.Name
 
-	if v2.defaultSet {
+	if v2.Default != nil {
 		result.Default = v2.Default
-		result.defaultSet = true
 	}
 	if v2.Description != "" {
 		result.Description = v2.Description
@@ -321,6 +328,27 @@ func (v *Variable) Merge(v2 *Variable) *Variable {
 	return &result
 }
 
+// Type returns the type of varialbe this is.
+func (v *Variable) Type() VariableType {
+	if v.Default == nil {
+		return VariableTypeString
+	}
+
+	var strVal string
+	if err := mapstructure.WeakDecode(v.Default, &strVal); err == nil {
+		v.Default = strVal
+		return VariableTypeString
+	}
+
+	var m map[string]string
+	if err := mapstructure.WeakDecode(v.Default, &m); err == nil {
+		v.Default = m
+		return VariableTypeMap
+	}
+
+	return VariableTypeUnknown
+}
+
 func (v *Variable) mergerName() string {
 	return v.Name
 }
@@ -331,59 +359,5 @@ func (v *Variable) mergerMerge(m merger) merger {
 
 // Required tests whether a variable is required or not.
 func (v *Variable) Required() bool {
-	return !v.defaultSet
-}
-
-func NewResourceVariable(key string) (*ResourceVariable, error) {
-	parts := strings.SplitN(key, ".", 3)
-	field := parts[2]
-	multi := false
-	var index int
-
-	if idx := strings.Index(field, "."); idx != -1 {
-		indexStr := field[:idx]
-		multi = indexStr == "*"
-		index = -1
-
-		if !multi {
-			indexInt, err := strconv.ParseInt(indexStr, 0, 0)
-			if err == nil {
-				multi = true
-				index = int(indexInt)
-			}
-		}
-
-		if multi {
-			field = field[idx+1:]
-		}
-	}
-
-	return &ResourceVariable{
-		Type:  parts[0],
-		Name:  parts[1],
-		Field: field,
-		Multi: multi,
-		Index: index,
-		key:   key,
-	}, nil
-}
-
-func (v *ResourceVariable) ResourceId() string {
-	return fmt.Sprintf("%s.%s", v.Type, v.Name)
-}
-
-func (v *ResourceVariable) FullKey() string {
-	return v.key
-}
-
-func NewUserVariable(key string) (*UserVariable, error) {
-	name := key[len("var."):]
-	return &UserVariable{
-		key:  key,
-		Name: name,
-	}, nil
-}
-
-func (v *UserVariable) FullKey() string {
-	return v.key
+	return v.Default == nil
 }
diff --git a/config/config_test.go b/config/config_test.go
index 4c66466b14..9b141fa8f5 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -2,6 +2,7 @@ package config
 
 import (
 	"path/filepath"
+	"reflect"
 	"testing"
 )
 
@@ -64,89 +65,17 @@ func TestConfigValidate_unknownVar(t *testing.T) {
 	}
 }
 
-func TestNewResourceVariable(t *testing.T) {
-	v, err := NewResourceVariable("foo.bar.baz")
-	if err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if v.Type != "foo" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if v.Name != "bar" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if v.Field != "baz" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if v.Multi {
-		t.Fatal("should not be multi")
-	}
-
-	if v.FullKey() != "foo.bar.baz" {
-		t.Fatalf("bad: %#v", v)
+func TestConfigValidate_varDefault(t *testing.T) {
+	c := testConfig(t, "validate-var-default")
+	if err := c.Validate(); err != nil {
+		t.Fatalf("should be valid: %s", err)
 	}
 }
 
-func TestResourceVariable_Multi(t *testing.T) {
-	v, err := NewResourceVariable("foo.bar.*.baz")
-	if err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if v.Type != "foo" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if v.Name != "bar" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if v.Field != "baz" {
-		t.Fatalf("bad: %#v", v)
-	}
-	if !v.Multi {
-		t.Fatal("should be multi")
-	}
-}
-
-func TestResourceVariable_MultiIndex(t *testing.T) {
-	cases := []struct {
-		Input string
-		Index int
-		Field string
-	}{
-		{"foo.bar.*.baz", -1, "baz"},
-		{"foo.bar.0.baz", 0, "baz"},
-		{"foo.bar.5.baz", 5, "baz"},
-	}
-
-	for _, tc := range cases {
-		v, err := NewResourceVariable(tc.Input)
-		if err != nil {
-			t.Fatalf("err: %s", err)
-		}
-		if !v.Multi {
-			t.Fatalf("should be multi: %s", tc.Input)
-		}
-		if v.Index != tc.Index {
-			t.Fatalf("bad: %d\n\n%s", v.Index, tc.Input)
-		}
-		if v.Field != tc.Field {
-			t.Fatalf("bad: %s\n\n%s", v.Field, tc.Input)
-		}
-	}
-}
-
-func TestNewUserVariable(t *testing.T) {
-	v, err := NewUserVariable("var.bar")
-	if err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if v.Name != "bar" {
-		t.Fatalf("bad: %#v", v.Name)
-	}
-	if v.FullKey() != "var.bar" {
-		t.Fatalf("bad: %#v", v)
+func TestConfigValidate_varDefaultBadType(t *testing.T) {
+	c := testConfig(t, "validate-var-default-bad-type")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
 	}
 }
 
@@ -164,6 +93,43 @@ func TestProviderConfigName(t *testing.T) {
 	}
 }
 
+func TestVariableDefaultsMap(t *testing.T) {
+	cases := []struct {
+		Default interface{}
+		Output  map[string]string
+	}{
+		{
+			nil,
+			nil,
+		},
+
+		{
+			"foo",
+			map[string]string{"var.foo": "foo"},
+		},
+
+		{
+			map[interface{}]interface{}{
+				"foo": "bar",
+				"bar": "baz",
+			},
+			map[string]string{
+				"var.foo":     "foo",
+				"var.foo.foo": "bar",
+				"var.foo.bar": "baz",
+			},
+		},
+	}
+
+	for i, tc := range cases {
+		v := &Variable{Name: "foo", Default: tc.Default}
+		actual := v.DefaultsMap()
+		if !reflect.DeepEqual(actual, tc.Output) {
+			t.Fatalf("%d: bad: %#v", i, actual)
+		}
+	}
+}
+
 func testConfig(t *testing.T, name string) *Config {
 	c, err := Load(filepath.Join(fixtureDir, name, "main.tf"))
 	if err != nil {
diff --git a/config/interpolate.go b/config/interpolate.go
new file mode 100644
index 0000000000..59f14a83bf
--- /dev/null
+++ b/config/interpolate.go
@@ -0,0 +1,267 @@
+package config
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// We really need to replace this with a real parser.
+var funcRegexp *regexp.Regexp = regexp.MustCompile(
+	`(?i)([a-z0-9_]+)\(\s*(?:([.a-z0-9_]+)\s*,\s*)*([.a-z0-9_]+)\s*\)`)
+
+// Interpolation is something that can be contained in a "${}" in a
+// configuration value.
+//
+// Interpolations might be simple variable references, or it might be
+// function calls, or even nested function calls.
+type Interpolation interface {
+	FullString() string
+	Interpolate(map[string]string) (string, error)
+	Variables() map[string]InterpolatedVariable
+}
+
+// InterpolationFunc is the function signature for implementing
+// callable functions in Terraform configurations.
+type InterpolationFunc func(map[string]string, ...string) (string, error)
+
+// An InterpolatedVariable is a variable reference within an interpolation.
+//
+// Implementations of this interface represents various sources where
+// variables can come from: user variables, resources, etc.
+type InterpolatedVariable interface {
+	FullKey() string
+}
+
+// FunctionInterpolation is an Interpolation that executes a function
+// with some variable number of arguments to generate a value.
+type FunctionInterpolation struct {
+	Func InterpolationFunc
+	Args []InterpolatedVariable
+
+	key string
+}
+
+// VariableInterpolation implements Interpolation for simple variable
+// interpolation. Ex: "${var.foo}" or "${aws_instance.foo.bar}"
+type VariableInterpolation struct {
+	Variable InterpolatedVariable
+
+	key string
+}
+
+// A ResourceVariable is a variable that is referencing the field
+// of a resource, such as "${aws_instance.foo.ami}"
+type ResourceVariable struct {
+	Type  string // Resource type, i.e. "aws_instance"
+	Name  string // Resource name
+	Field string // Resource field
+
+	Multi bool // True if multi-variable: aws_instance.foo.*.id
+	Index int  // Index for multi-variable: aws_instance.foo.1.id == 1
+
+	key string
+}
+
+// A UserVariable is a variable that is referencing a user variable
+// that is inputted from outside the configuration. This looks like
+// "${var.foo}"
+type UserVariable struct {
+	Name string
+	Elem string
+
+	key string
+}
+
+// NewInterpolation takes some string and returns the valid
+// Interpolation associated with it, or error if a valid
+// interpolation could not be found or the interpolation itself
+// is invalid.
+func NewInterpolation(v string) (Interpolation, error) {
+	match := funcRegexp.FindStringSubmatch(v)
+	if match != nil {
+		fn, ok := Funcs[match[1]]
+		if !ok {
+			return nil, fmt.Errorf(
+				"%s: Unknown function '%s'",
+				v, match[1])
+		}
+
+		args := make([]InterpolatedVariable, 0, len(match)-2)
+		for i := 2; i < len(match); i++ {
+			// This can be empty if we have a single argument
+			// due to the format of the regexp.
+			if match[i] == "" {
+				continue
+			}
+
+			v, err := NewInterpolatedVariable(match[i])
+			if err != nil {
+				return nil, err
+			}
+
+			args = append(args, v)
+		}
+
+		return &FunctionInterpolation{
+			Func: fn,
+			Args: args,
+
+			key: v,
+		}, nil
+	}
+
+	if idx := strings.Index(v, "."); idx >= 0 {
+		v, err := NewInterpolatedVariable(v)
+		if err != nil {
+			return nil, err
+		}
+
+		return &VariableInterpolation{
+			Variable: v,
+			key:      v.FullKey(),
+		}, nil
+	}
+
+	return nil, fmt.Errorf(
+		"Interpolation '%s' is not a valid interpolation. " +
+			"Please check your syntax and try again.")
+}
+
+func NewInterpolatedVariable(v string) (InterpolatedVariable, error) {
+	if !strings.HasPrefix(v, "var.") {
+		return NewResourceVariable(v)
+	}
+
+	return NewUserVariable(v)
+}
+
+func (i *FunctionInterpolation) FullString() string {
+	return i.key
+}
+
+func (i *FunctionInterpolation) Interpolate(
+	vs map[string]string) (string, error) {
+	args := make([]string, len(i.Args))
+	for idx, a := range i.Args {
+		k := a.FullKey()
+		v, ok := vs[k]
+		if !ok {
+			return "", fmt.Errorf(
+				"%s: variable argument value unknown: %s",
+				i.FullString(),
+				k)
+		}
+
+		args[idx] = v
+	}
+
+	return i.Func(vs, args...)
+}
+
+func (i *FunctionInterpolation) Variables() map[string]InterpolatedVariable {
+	result := make(map[string]InterpolatedVariable)
+	for _, a := range i.Args {
+		k := a.FullKey()
+		if _, ok := result[k]; ok {
+			continue
+		}
+
+		result[k] = a
+	}
+
+	return result
+}
+
+func (i *VariableInterpolation) FullString() string {
+	return i.key
+}
+
+func (i *VariableInterpolation) Interpolate(
+	vs map[string]string) (string, error) {
+	v, ok := vs[i.key]
+	if !ok {
+		return "", fmt.Errorf(
+			"%s: value for variable not found",
+			i.key)
+	}
+
+	return v, nil
+}
+
+func (i *VariableInterpolation) Variables() map[string]InterpolatedVariable {
+	return map[string]InterpolatedVariable{i.key: i.Variable}
+}
+
+func NewResourceVariable(key string) (*ResourceVariable, error) {
+	parts := strings.SplitN(key, ".", 3)
+	if len(parts) < 3 {
+		return nil, fmt.Errorf(
+			"%s: resource variables must be three parts: type.name.attr",
+			key)
+	}
+
+	field := parts[2]
+	multi := false
+	var index int
+
+	if idx := strings.Index(field, "."); idx != -1 {
+		indexStr := field[:idx]
+		multi = indexStr == "*"
+		index = -1
+
+		if !multi {
+			indexInt, err := strconv.ParseInt(indexStr, 0, 0)
+			if err == nil {
+				multi = true
+				index = int(indexInt)
+			}
+		}
+
+		if multi {
+			field = field[idx+1:]
+		}
+	}
+
+	return &ResourceVariable{
+		Type:  parts[0],
+		Name:  parts[1],
+		Field: field,
+		Multi: multi,
+		Index: index,
+		key:   key,
+	}, nil
+}
+
+func (v *ResourceVariable) ResourceId() string {
+	return fmt.Sprintf("%s.%s", v.Type, v.Name)
+}
+
+func (v *ResourceVariable) FullKey() string {
+	return v.key
+}
+
+func NewUserVariable(key string) (*UserVariable, error) {
+	name := key[len("var."):]
+	elem := ""
+	if idx := strings.Index(name, "."); idx > -1 {
+		elem = name[idx+1:]
+		name = name[:idx]
+	}
+
+	return &UserVariable{
+		key: key,
+
+		Name: name,
+		Elem: elem,
+	}, nil
+}
+
+func (v *UserVariable) FullKey() string {
+	return v.key
+}
+
+func (v *UserVariable) GoString() string {
+	return fmt.Sprintf("*%#v", *v)
+}
diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
new file mode 100644
index 0000000000..9c076db2d3
--- /dev/null
+++ b/config/interpolate_funcs.go
@@ -0,0 +1,35 @@
+package config
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Funcs is the mapping of built-in functions for configuration.
+var Funcs map[string]InterpolationFunc
+
+func init() {
+	Funcs = map[string]InterpolationFunc{
+		"lookup": interpolationFuncLookup,
+	}
+}
+
+// interpolationFuncLookup implements the "lookup" function that allows
+// dynamic lookups of map types within a Terraform configuration.
+func interpolationFuncLookup(
+	vs map[string]string, args ...string) (string, error) {
+	if len(args) != 2 {
+		return "", fmt.Errorf(
+			"lookup expects 2 arguments, got %d", len(args))
+	}
+
+	k := fmt.Sprintf("var.%s", strings.Join(args, "."))
+	v, ok := vs[k]
+	if !ok {
+		return "", fmt.Errorf(
+			"lookup in '%s' failed to find '%s'",
+			args[0], args[1])
+	}
+
+	return v, nil
+}
diff --git a/config/interpolate_funcs_test.go b/config/interpolate_funcs_test.go
new file mode 100644
index 0000000000..20f964c438
--- /dev/null
+++ b/config/interpolate_funcs_test.go
@@ -0,0 +1,54 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestInterpolateFuncLookup(t *testing.T) {
+	cases := []struct {
+		M      map[string]string
+		Args   []string
+		Result string
+		Error  bool
+	}{
+		{
+			map[string]string{
+				"var.foo.bar": "baz",
+			},
+			[]string{"foo", "bar"},
+			"baz",
+			false,
+		},
+
+		// Invalid key
+		{
+			map[string]string{
+				"var.foo.bar": "baz",
+			},
+			[]string{"foo", "baz"},
+			"",
+			true,
+		},
+
+		// Too many args
+		{
+			map[string]string{
+				"var.foo.bar": "baz",
+			},
+			[]string{"foo", "bar", "baz"},
+			"",
+			true,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := interpolationFuncLookup(tc.M, tc.Args...)
+		if (err != nil) != tc.Error {
+			t.Fatalf("%d: err: %s", i, err)
+		}
+
+		if actual != tc.Result {
+			t.Fatalf("%d: bad: %#v", i, actual)
+		}
+	}
+}
diff --git a/config/interpolate_test.go b/config/interpolate_test.go
new file mode 100644
index 0000000000..3b01c6cd79
--- /dev/null
+++ b/config/interpolate_test.go
@@ -0,0 +1,303 @@
+package config
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestNewInterpolation(t *testing.T) {
+	cases := []struct {
+		Input  string
+		Result Interpolation
+		Error  bool
+	}{
+		{
+			"foo",
+			nil,
+			true,
+		},
+
+		{
+			"var.foo",
+			&VariableInterpolation{
+				Variable: &UserVariable{
+					Name: "foo",
+					key:  "var.foo",
+				},
+				key: "var.foo",
+			},
+			false,
+		},
+
+		{
+			"lookup(var.foo, var.bar)",
+			&FunctionInterpolation{
+				Func: nil, // Funcs["lookup"]
+				Args: []InterpolatedVariable{
+					&UserVariable{
+						Name: "foo",
+						key:  "var.foo",
+					},
+					&UserVariable{
+						Name: "bar",
+						key:  "var.bar",
+					},
+				},
+				key: "lookup(var.foo, var.bar)",
+			},
+			false,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := NewInterpolation(tc.Input)
+		if (err != nil) != tc.Error {
+			t.Fatalf("%d. Error: %s", i, err)
+		}
+
+		// This is jank, but reflect.DeepEqual never has functions
+		// being the same.
+		if f, ok := actual.(*FunctionInterpolation); ok {
+			f.Func = nil
+		}
+
+		if !reflect.DeepEqual(actual, tc.Result) {
+			t.Fatalf("%d bad: %#v", i, actual)
+		}
+	}
+}
+
+func TestNewInterpolatedVariable(t *testing.T) {
+	cases := []struct {
+		Input  string
+		Result InterpolatedVariable
+		Error  bool
+	}{
+		{
+			"var.foo",
+			&UserVariable{
+				Name: "foo",
+				key:  "var.foo",
+			},
+			false,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := NewInterpolatedVariable(tc.Input)
+		if (err != nil) != tc.Error {
+			t.Fatalf("%d. Error: %s", i, err)
+		}
+		if !reflect.DeepEqual(actual, tc.Result) {
+			t.Fatalf("%d bad: %#v", i, actual)
+		}
+	}
+}
+
+func TestNewResourceVariable(t *testing.T) {
+	v, err := NewResourceVariable("foo.bar.baz")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if v.Type != "foo" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Name != "bar" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Field != "baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Multi {
+		t.Fatal("should not be multi")
+	}
+
+	if v.FullKey() != "foo.bar.baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+}
+
+func TestNewUserVariable(t *testing.T) {
+	v, err := NewUserVariable("var.bar")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if v.Name != "bar" {
+		t.Fatalf("bad: %#v", v.Name)
+	}
+	if v.FullKey() != "var.bar" {
+		t.Fatalf("bad: %#v", v)
+	}
+}
+
+func TestNewUserVariable_map(t *testing.T) {
+	v, err := NewUserVariable("var.bar.baz")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if v.Name != "bar" {
+		t.Fatalf("bad: %#v", v.Name)
+	}
+	if v.Elem != "baz" {
+		t.Fatalf("bad: %#v", v.Elem)
+	}
+	if v.FullKey() != "var.bar.baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+}
+
+func TestFunctionInterpolation_impl(t *testing.T) {
+	var _ Interpolation = new(FunctionInterpolation)
+}
+
+func TestFunctionInterpolation(t *testing.T) {
+	v1, err := NewInterpolatedVariable("var.foo")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	v2, err := NewInterpolatedVariable("var.bar")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	fn := func(vs map[string]string, args ...string) (string, error) {
+		return strings.Join(args, " "), nil
+	}
+
+	i := &FunctionInterpolation{
+		Func: fn,
+		Args: []InterpolatedVariable{v1, v2},
+		key:  "foo",
+	}
+	if i.FullString() != "foo" {
+		t.Fatalf("err: %#v", i)
+	}
+
+	expected := map[string]InterpolatedVariable{
+		"var.foo": v1,
+		"var.bar": v2,
+	}
+	if !reflect.DeepEqual(i.Variables(), expected) {
+		t.Fatalf("bad: %#v", i.Variables())
+	}
+
+	actual, err := i.Interpolate(map[string]string{
+		"var.foo": "bar",
+		"var.bar": "baz",
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if actual != "bar baz" {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
+func TestResourceVariable_impl(t *testing.T) {
+	var _ InterpolatedVariable = new(ResourceVariable)
+}
+
+func TestResourceVariable_Multi(t *testing.T) {
+	v, err := NewResourceVariable("foo.bar.*.baz")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if v.Type != "foo" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Name != "bar" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if v.Field != "baz" {
+		t.Fatalf("bad: %#v", v)
+	}
+	if !v.Multi {
+		t.Fatal("should be multi")
+	}
+}
+
+func TestResourceVariable_MultiIndex(t *testing.T) {
+	cases := []struct {
+		Input string
+		Index int
+		Field string
+	}{
+		{"foo.bar.*.baz", -1, "baz"},
+		{"foo.bar.0.baz", 0, "baz"},
+		{"foo.bar.5.baz", 5, "baz"},
+	}
+
+	for _, tc := range cases {
+		v, err := NewResourceVariable(tc.Input)
+		if err != nil {
+			t.Fatalf("err: %s", err)
+		}
+		if !v.Multi {
+			t.Fatalf("should be multi: %s", tc.Input)
+		}
+		if v.Index != tc.Index {
+			t.Fatalf("bad: %d\n\n%s", v.Index, tc.Input)
+		}
+		if v.Field != tc.Field {
+			t.Fatalf("bad: %s\n\n%s", v.Field, tc.Input)
+		}
+	}
+}
+
+func TestUserVariable_impl(t *testing.T) {
+	var _ InterpolatedVariable = new(UserVariable)
+}
+
+func TestVariableInterpolation_impl(t *testing.T) {
+	var _ Interpolation = new(VariableInterpolation)
+}
+
+func TestVariableInterpolation(t *testing.T) {
+	uv, err := NewUserVariable("var.foo")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	i := &VariableInterpolation{Variable: uv, key: "var.foo"}
+	if i.FullString() != "var.foo" {
+		t.Fatalf("err: %#v", i)
+	}
+
+	expected := map[string]InterpolatedVariable{"var.foo": uv}
+	if !reflect.DeepEqual(i.Variables(), expected) {
+		t.Fatalf("bad: %#v", i.Variables())
+	}
+
+	actual, err := i.Interpolate(map[string]string{
+		"var.foo": "bar",
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if actual != "bar" {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
+func TestVariableInterpolation_missing(t *testing.T) {
+	uv, err := NewUserVariable("var.foo")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	i := &VariableInterpolation{Variable: uv, key: "var.foo"}
+	_, err = i.Interpolate(map[string]string{
+		"var.bar": "bar",
+	})
+	if err == nil {
+		t.Fatal("should error")
+	}
+}
diff --git a/config/interpolate_walk.go b/config/interpolate_walk.go
new file mode 100644
index 0000000000..fd8a708330
--- /dev/null
+++ b/config/interpolate_walk.go
@@ -0,0 +1,151 @@
+package config
+
+import (
+	"reflect"
+	"regexp"
+	"strings"
+
+	"github.com/mitchellh/reflectwalk"
+)
+
+// interpRegexp is a regexp that matches interpolations such as ${foo.bar}
+var interpRegexp *regexp.Regexp = regexp.MustCompile(
+	`(?i)(\$+)\{([\s*-.,\(\)a-z0-9_]+)\}`)
+
+// interpolationWalker implements interfaces for the reflectwalk package
+// (github.com/mitchellh/reflectwalk) that can be used to automatically
+// execute a callback for an interpolation.
+type interpolationWalker struct {
+	F       interpolationWalkerFunc
+	Replace bool
+
+	key         []string
+	loc         reflectwalk.Location
+	cs          []reflect.Value
+	csData      interface{}
+	unknownKeys []string
+}
+
+// interpolationWalkerFunc is the callback called by interpolationWalk.
+// It is called with any interpolation found. It should return a value
+// to replace the interpolation with, along with any errors.
+//
+// If Replace is set to false in interpolationWalker, then the replace
+// value can be anything as it will have no effect.
+type interpolationWalkerFunc func(Interpolation) (string, error)
+
+func (w *interpolationWalker) Enter(loc reflectwalk.Location) error {
+	w.loc = loc
+	return nil
+}
+
+func (w *interpolationWalker) Exit(loc reflectwalk.Location) error {
+	w.loc = reflectwalk.None
+
+	switch loc {
+	case reflectwalk.Map:
+		w.cs = w.cs[:len(w.cs)-1]
+	case reflectwalk.MapValue:
+		w.key = w.key[:len(w.key)-1]
+	}
+
+	return nil
+}
+
+func (w *interpolationWalker) Map(m reflect.Value) error {
+	w.cs = append(w.cs, m)
+	return nil
+}
+
+func (w *interpolationWalker) MapElem(m, k, v reflect.Value) error {
+	w.csData = k
+	w.key = append(w.key, k.String())
+	return nil
+}
+
+func (w *interpolationWalker) Primitive(v reflect.Value) error {
+	setV := v
+
+	// We only care about strings
+	if v.Kind() == reflect.Interface {
+		setV = v
+		v = v.Elem()
+	}
+	if v.Kind() != reflect.String {
+		return nil
+	}
+
+	// XXX: This can be a lot more efficient if we used a real
+	// parser. A regexp is a hammer though that will get this working.
+
+	matches := interpRegexp.FindAllStringSubmatch(v.String(), -1)
+	if len(matches) == 0 {
+		return nil
+	}
+
+	result := v.String()
+	for _, match := range matches {
+		dollars := len(match[1])
+
+		// If there are even amounts of dollar signs, then it is escaped
+		if dollars%2 == 0 {
+			continue
+		}
+
+		// Interpolation found, instantiate it
+		key := match[2]
+
+		i, err := NewInterpolation(key)
+		if err != nil {
+			return err
+		}
+
+		replaceVal, err := w.F(i)
+		if err != nil {
+			return err
+		}
+
+		if w.Replace {
+			// If this is an unknown variable, then we remove it from
+			// the configuration.
+			if replaceVal == UnknownVariableValue {
+				w.removeCurrent()
+				return nil
+			}
+
+			result = strings.Replace(result, match[0], replaceVal, -1)
+		}
+	}
+
+	if w.Replace {
+		resultVal := reflect.ValueOf(result)
+		if w.loc == reflectwalk.MapValue {
+			// If we're in a map, then the only way to set a map value is
+			// to set it directly.
+			m := w.cs[len(w.cs)-1]
+			mk := w.csData.(reflect.Value)
+			m.SetMapIndex(mk, resultVal)
+		} else {
+			// Otherwise, we should be addressable
+			setV.Set(resultVal)
+		}
+	}
+
+	return nil
+}
+
+func (w *interpolationWalker) removeCurrent() {
+	c := w.cs[len(w.cs)-1]
+	switch c.Kind() {
+	case reflect.Map:
+		// Zero value so that we delete the map key
+		var val reflect.Value
+
+		// Get the key and delete it
+		k := w.csData.(reflect.Value)
+		c.SetMapIndex(k, val)
+	}
+
+	// Append the key to the unknown keys
+	w.unknownKeys = append(w.unknownKeys, strings.Join(w.key, "."))
+}
diff --git a/config/interpolate_walk_test.go b/config/interpolate_walk_test.go
new file mode 100644
index 0000000000..a731f8615e
--- /dev/null
+++ b/config/interpolate_walk_test.go
@@ -0,0 +1,121 @@
+package config
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/mitchellh/reflectwalk"
+)
+
+func TestInterpolationWalker_detect(t *testing.T) {
+	cases := []struct {
+		Input  interface{}
+		Result []Interpolation
+	}{
+		{
+			Input: map[string]interface{}{
+				"foo": "$${var.foo}",
+			},
+			Result: nil,
+		},
+
+		{
+			Input: map[string]interface{}{
+				"foo": "${var.foo}",
+			},
+			Result: []Interpolation{
+				&VariableInterpolation{
+					Variable: &UserVariable{
+						Name: "foo",
+						key:  "var.foo",
+					},
+					key: "var.foo",
+				},
+			},
+		},
+
+		{
+			Input: map[string]interface{}{
+				"foo": "${lookup(var.foo)}",
+			},
+			Result: []Interpolation{
+				&FunctionInterpolation{
+					Func: nil,
+					Args: []InterpolatedVariable{
+						&UserVariable{
+							Name: "foo",
+							key:  "var.foo",
+						},
+					},
+					key: "lookup(var.foo)",
+				},
+			},
+		},
+	}
+
+	for i, tc := range cases {
+		var actual []Interpolation
+
+		detectFn := func(i Interpolation) (string, error) {
+			actual = append(actual, i)
+			return "", nil
+		}
+
+		w := &interpolationWalker{F: detectFn}
+		if err := reflectwalk.Walk(tc.Input, w); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		for _, a := range actual {
+			// This is jank, but reflect.DeepEqual never has functions
+			// being the same.
+			if f, ok := a.(*FunctionInterpolation); ok {
+				f.Func = nil
+			}
+		}
+
+		if !reflect.DeepEqual(actual, tc.Result) {
+			t.Fatalf("%d: bad:\n\n%#v", i, actual)
+		}
+	}
+}
+
+func TestInterpolationWalker_replace(t *testing.T) {
+	cases := []struct {
+		Input  interface{}
+		Output interface{}
+	}{
+		{
+			Input: map[string]interface{}{
+				"foo": "$${var.foo}",
+			},
+			Output: map[string]interface{}{
+				"foo": "$${var.foo}",
+			},
+		},
+
+		{
+			Input: map[string]interface{}{
+				"foo": "hello, ${var.foo}",
+			},
+			Output: map[string]interface{}{
+				"foo": "hello, bar",
+			},
+		},
+	}
+
+	for i, tc := range cases {
+		fn := func(i Interpolation) (string, error) {
+			return "bar", nil
+		}
+
+		w := &interpolationWalker{F: fn, Replace: true}
+		if err := reflectwalk.Walk(tc.Input, w); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+
+		if !reflect.DeepEqual(tc.Input, tc.Output) {
+			t.Fatalf("%d: bad:\n\n%#v", i, tc.Input)
+		}
+	}
+}
diff --git a/config/loader_libucl.go b/config/loader_libucl.go
index c6050d9225..cd14774fd7 100644
--- a/config/loader_libucl.go
+++ b/config/loader_libucl.go
@@ -30,7 +30,7 @@ func (t *libuclConfigurable) Config() (*Config, error) {
 	}
 
 	type LibuclVariable struct {
-		Default     string
+		Default     interface{}
 		Description string
 		Fields      []string `libucl:",decodedFields"`
 	}
@@ -51,20 +51,27 @@ func (t *libuclConfigurable) Config() (*Config, error) {
 	if len(rawConfig.Variable) > 0 {
 		config.Variables = make([]*Variable, 0, len(rawConfig.Variable))
 		for k, v := range rawConfig.Variable {
-			defaultSet := false
-			for _, f := range v.Fields {
-				if f == "Default" {
-					defaultSet = true
-					break
+			// Defaults turn into a slice of map[string]interface{} and
+			// we need to make sure to convert that down into the
+			// proper type for Config.
+			if ms, ok := v.Default.([]map[string]interface{}); ok {
+				def := make(map[string]interface{})
+				for _, m := range ms {
+					for k, v := range m {
+						def[k] = v
+					}
 				}
+
+				v.Default = def
 			}
 
-			config.Variables = append(config.Variables, &Variable{
+			newVar := &Variable{
 				Name:        k,
 				Default:     v.Default,
 				Description: v.Description,
-				defaultSet:  defaultSet,
-			})
+			}
+
+			config.Variables = append(config.Variables, newVar)
 		}
 	}
 
diff --git a/config/loader_test.go b/config/loader_test.go
index 658e1c2489..0cb8183cfa 100644
--- a/config/loader_test.go
+++ b/config/loader_test.go
@@ -437,20 +437,20 @@ func variablesStr(vs []*Variable) string {
 	for _, k := range ks {
 		v := m[k]
 
-		if v.Default == "" {
+		required := ""
+		if v.Required() {
+			required = " (required)"
+		}
+
+		if v.Default == nil || v.Default == "" {
 			v.Default = "<>"
 		}
 		if v.Description == "" {
 			v.Description = "<>"
 		}
 
-		required := ""
-		if v.Required() {
-			required = " (required)"
-		}
-
 		result += fmt.Sprintf(
-			"%s%s\n  %s\n  %s\n",
+			"%s%s\n  %v\n  %s\n",
 			k,
 			required,
 			v.Default,
diff --git a/config/merge_test.go b/config/merge_test.go
index fb52677b57..10ef5f37a7 100644
--- a/config/merge_test.go
+++ b/config/merge_test.go
@@ -103,7 +103,7 @@ func TestMerge(t *testing.T) {
 					&Resource{Name: "bar"},
 				},
 				Variables: []*Variable{
-					&Variable{Name: "foo", Default: "bar", defaultSet: true},
+					&Variable{Name: "foo", Default: "bar"},
 					&Variable{Name: "bar"},
 				},
 
@@ -124,7 +124,7 @@ func TestMerge(t *testing.T) {
 					&Resource{Name: "bar"},
 				},
 				Variables: []*Variable{
-					&Variable{Name: "foo", Default: "bar", defaultSet: true},
+					&Variable{Name: "foo", Default: "bar"},
 					&Variable{Name: "foo"},
 					&Variable{Name: "bar"},
 				},
diff --git a/config/raw_config.go b/config/raw_config.go
index f16921fd1f..c97b29c527 100644
--- a/config/raw_config.go
+++ b/config/raw_config.go
@@ -69,26 +69,43 @@ func (r *RawConfig) Interpolate(vs map[string]string) error {
 	if err != nil {
 		return err
 	}
-
-	w := &variableReplaceWalker{Values: vs}
 	r.config = config.(map[string]interface{})
+
+	fn := func(i Interpolation) (string, error) {
+		return i.Interpolate(vs)
+	}
+
+	w := &interpolationWalker{F: fn, Replace: true}
 	err = reflectwalk.Walk(r.config, w)
 	if err != nil {
 		return err
 	}
 
-	r.unknownKeys = w.UnknownKeys
+	r.unknownKeys = w.unknownKeys
 	return nil
 }
 
 func (r *RawConfig) init() error {
-	walker := new(variableDetectWalker)
+	r.config = r.Raw
+	r.Variables = nil
+
+	fn := func(i Interpolation) (string, error) {
+		for k, v := range i.Variables() {
+			if r.Variables == nil {
+				r.Variables = make(map[string]InterpolatedVariable)
+			}
+
+			r.Variables[k] = v
+		}
+
+		return "", nil
+	}
+
+	walker := &interpolationWalker{F: fn}
 	if err := reflectwalk.Walk(r.Raw, walker); err != nil {
 		return err
 	}
 
-	r.Variables = walker.Variables
-	r.config = r.Raw
 	return nil
 }
 
diff --git a/config/test-fixtures/validate-good/main.tf b/config/test-fixtures/validate-good/main.tf
index 7cd2296117..965877a104 100644
--- a/config/test-fixtures/validate-good/main.tf
+++ b/config/test-fixtures/validate-good/main.tf
@@ -3,6 +3,12 @@ variable "foo" {
     description = "bar";
 }
 
+variable "amis" {
+    default = {
+        "east": "foo",
+    }
+}
+
 provider "aws" {
   access_key = "foo";
   secret_key = "bar";
@@ -16,7 +22,7 @@ resource "aws_security_group" "firewall" {
 }
 
 resource aws_instance "web" {
-    ami = "${var.foo}"
+    ami = "${var.amis.east}"
     security_groups = [
         "foo",
         "${aws_security_group.firewall.foo}"
diff --git a/config/test-fixtures/validate-var-default-bad-type/main.tf b/config/test-fixtures/validate-var-default-bad-type/main.tf
new file mode 100644
index 0000000000..c7975db387
--- /dev/null
+++ b/config/test-fixtures/validate-var-default-bad-type/main.tf
@@ -0,0 +1,3 @@
+variable "foo" {
+  default = ["foo", "bar"]
+}
diff --git a/config/test-fixtures/validate-var-default/main.tf b/config/test-fixtures/validate-var-default/main.tf
new file mode 100644
index 0000000000..75a6bc9e0a
--- /dev/null
+++ b/config/test-fixtures/validate-var-default/main.tf
@@ -0,0 +1,9 @@
+variable "foo" {
+  default = "bar"
+}
+
+variable "foo" {
+  default = {
+    "foo" = "bar"
+  }
+}
diff --git a/config/variable.go b/config/variable.go
deleted file mode 100644
index 40a3b28df5..0000000000
--- a/config/variable.go
+++ /dev/null
@@ -1,216 +0,0 @@
-package config
-
-import (
-	"fmt"
-	"reflect"
-	"regexp"
-	"strings"
-
-	"github.com/mitchellh/reflectwalk"
-)
-
-// varRegexp is a regexp that matches variables such as ${foo.bar}
-var varRegexp *regexp.Regexp
-
-func init() {
-	varRegexp = regexp.MustCompile(`(?i)(\$+)\{([*-.a-z0-9_]+)\}`)
-}
-
-// ReplaceVariables takes a configuration and a mapping of variables
-// and performs the structure walking necessary to properly replace
-// all the variables.
-func ReplaceVariables(
-	c interface{},
-	vs map[string]string) ([]string, error) {
-	w := &variableReplaceWalker{Values: vs}
-	if err := reflectwalk.Walk(c, w); err != nil {
-		return nil, err
-	}
-
-	return w.UnknownKeys, nil
-}
-
-// variableDetectWalker implements interfaces for the reflectwalk package
-// (github.com/mitchellh/reflectwalk) that can be used to automatically
-// pull out the variables that need replacing.
-type variableDetectWalker struct {
-	Variables map[string]InterpolatedVariable
-}
-
-func (w *variableDetectWalker) Primitive(v reflect.Value) error {
-	// We only care about strings
-	if v.Kind() == reflect.Interface {
-		v = v.Elem()
-	}
-	if v.Kind() != reflect.String {
-		return nil
-	}
-
-	// XXX: This can be a lot more efficient if we used a real
-	// parser. A regexp is a hammer though that will get this working.
-
-	matches := varRegexp.FindAllStringSubmatch(v.String(), -1)
-	if len(matches) == 0 {
-		return nil
-	}
-
-	for _, match := range matches {
-		dollars := len(match[1])
-
-		// If there are even amounts of dollar signs, then it is escaped
-		if dollars%2 == 0 {
-			continue
-		}
-
-		// Otherwise, record it
-		key := match[2]
-		if w.Variables == nil {
-			w.Variables = make(map[string]InterpolatedVariable)
-		}
-		if _, ok := w.Variables[key]; ok {
-			continue
-		}
-
-		var err error
-		var iv InterpolatedVariable
-		if strings.Index(key, ".") == -1 {
-			return fmt.Errorf(
-				"Interpolated variable '%s' has bad format. "+
-					"Did you mean 'var.%s'?",
-				key, key)
-		}
-
-		if strings.HasPrefix(key, "var.") {
-			iv, err = NewUserVariable(key)
-		} else {
-			iv, err = NewResourceVariable(key)
-		}
-
-		if err != nil {
-			return err
-		}
-
-		w.Variables[key] = iv
-	}
-
-	return nil
-}
-
-// variableReplaceWalker implements interfaces for reflectwalk that
-// is used to replace variables with their values.
-//
-// If Values does not have every available value, then the program
-// will _panic_. The variableDetectWalker will tell you all variables
-// you need.
-type variableReplaceWalker struct {
-	Values      map[string]string
-	UnknownKeys []string
-
-	key    []string
-	loc    reflectwalk.Location
-	cs     []reflect.Value
-	csData interface{}
-}
-
-func (w *variableReplaceWalker) Enter(loc reflectwalk.Location) error {
-	w.loc = loc
-	return nil
-}
-
-func (w *variableReplaceWalker) Exit(loc reflectwalk.Location) error {
-	w.loc = reflectwalk.None
-
-	switch loc {
-	case reflectwalk.Map:
-		w.cs = w.cs[:len(w.cs)-1]
-	case reflectwalk.MapValue:
-		w.key = w.key[:len(w.key)-1]
-	}
-
-	return nil
-}
-
-func (w *variableReplaceWalker) Map(m reflect.Value) error {
-	w.cs = append(w.cs, m)
-	return nil
-}
-
-func (w *variableReplaceWalker) MapElem(m, k, v reflect.Value) error {
-	w.csData = k
-	w.key = append(w.key, k.String())
-	return nil
-}
-
-func (w *variableReplaceWalker) Primitive(v reflect.Value) error {
-	// We only care about strings
-	setV := v
-	if v.Kind() == reflect.Interface {
-		setV = v
-		v = v.Elem()
-	}
-	if v.Kind() != reflect.String {
-		return nil
-	}
-
-	matches := varRegexp.FindAllStringSubmatch(v.String(), -1)
-	if len(matches) == 0 {
-		return nil
-	}
-
-	result := v.String()
-	for _, match := range matches {
-		dollars := len(match[1])
-
-		// If there are even amounts of dollar signs, then it is escaped
-		if dollars%2 == 0 {
-			continue
-		}
-
-		// Get the key
-		key := match[2]
-		value, ok := w.Values[key]
-		if !ok {
-			panic("no value for variable key: " + key)
-		}
-
-		// If this is an unknown variable, then we remove it from
-		// the configuration.
-		if value == UnknownVariableValue {
-			w.removeCurrent()
-			return nil
-		}
-
-		// Replace
-		result = strings.Replace(result, match[0], value, -1)
-	}
-
-	resultVal := reflect.ValueOf(result)
-	if w.loc == reflectwalk.MapValue {
-		// If we're in a map, then the only way to set a map value is
-		// to set it directly.
-		m := w.cs[len(w.cs)-1]
-		mk := w.csData.(reflect.Value)
-		m.SetMapIndex(mk, resultVal)
-	} else {
-		// Otherwise, we should be addressable
-		setV.Set(resultVal)
-	}
-
-	return nil
-}
-
-func (w *variableReplaceWalker) removeCurrent() {
-	c := w.cs[len(w.cs)-1]
-	switch c.Kind() {
-	case reflect.Map:
-		// Zero value so that we delete the map key
-		var val reflect.Value
-
-		// Get the key and delete it
-		k := w.csData.(reflect.Value)
-		c.SetMapIndex(k, val)
-	}
-
-	// Append the key to the unknown keys
-	w.UnknownKeys = append(w.UnknownKeys, strings.Join(w.key, "."))
-}
diff --git a/config/variable_test.go b/config/variable_test.go
deleted file mode 100644
index 7e3b5fc110..0000000000
--- a/config/variable_test.go
+++ /dev/null
@@ -1,282 +0,0 @@
-package config
-
-import (
-	"reflect"
-	"testing"
-
-	"github.com/mitchellh/reflectwalk"
-)
-
-func BenchmarkVariableDetectWalker(b *testing.B) {
-	w := new(variableDetectWalker)
-	str := reflect.ValueOf(`foo ${var.bar} bar ${bar.baz.bing} $${escaped}`)
-
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		w.Variables = nil
-		w.Primitive(str)
-	}
-}
-
-func BenchmarkVariableReplaceWalker(b *testing.B) {
-	w := &variableReplaceWalker{
-		Values: map[string]string{
-			"var.bar":      "bar",
-			"bar.baz.bing": "baz",
-		},
-	}
-
-	str := `foo ${var.bar} bar ${bar.baz.bing} $${escaped}`
-
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		if err := reflectwalk.Walk(&str, w); err != nil {
-			panic(err)
-		}
-	}
-}
-
-func TestReplaceVariables(t *testing.T) {
-	input := "foo-${var.bar}"
-	expected := "foo-bar"
-
-	unk, err := ReplaceVariables(&input, map[string]string{
-		"var.bar": "bar",
-	})
-	if err != nil {
-		t.Fatalf("err: %s", err)
-	}
-	if len(unk) > 0 {
-		t.Fatal("bad: %#v", unk)
-	}
-
-	if input != expected {
-		t.Fatalf("bad: %#v", input)
-	}
-}
-
-func TestVariableDetectWalker(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo ${var.bar}`
-	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if len(w.Variables) != 1 {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-	if w.Variables["var.bar"].(*UserVariable).FullKey() != "var.bar" {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-}
-
-func TestVariableDetectWalker_resource(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo ${ec2.foo.bar}`
-	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if len(w.Variables) != 1 {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-	if w.Variables["ec2.foo.bar"].(*ResourceVariable).FullKey() != "ec2.foo.bar" {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-}
-
-func TestVariableDetectWalker_resourceMulti(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo ${ec2.foo.*.bar}`
-	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if len(w.Variables) != 1 {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-	if w.Variables["ec2.foo.*.bar"].(*ResourceVariable).FullKey() != "ec2.foo.*.bar" {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-}
-
-func TestVariableDetectWalker_bad(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo ${bar}`
-	if err := w.Primitive(reflect.ValueOf(str)); err == nil {
-		t.Fatal("should error")
-	}
-}
-
-func TestVariableDetectWalker_escaped(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo $${var.bar}`
-	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if len(w.Variables) > 0 {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-}
-
-func TestVariableDetectWalker_empty(t *testing.T) {
-	w := new(variableDetectWalker)
-
-	str := `foo`
-	if err := w.Primitive(reflect.ValueOf(str)); err != nil {
-		t.Fatalf("err: %s", err)
-	}
-
-	if len(w.Variables) > 0 {
-		t.Fatalf("bad: %#v", w.Variables)
-	}
-}
-
-func TestVariableReplaceWalker(t *testing.T) {
-	w := &variableReplaceWalker{
-		Values: map[string]string{
-			"var.bar":     "bar",
-			"var.unknown": UnknownVariableValue,
-		},
-	}
-
-	cases := []struct {
-		Input  interface{}
-		Output interface{}
-	}{
-		{
-			`foo ${var.bar}`,
-			"foo bar",
-		},
-		{
-			[]string{"foo", "${var.bar}"},
-			[]string{"foo", "bar"},
-		},
-		{
-			map[string]interface{}{
-				"ami": "${var.bar}",
-				"security_groups": []interface{}{
-					"foo",
-					"${var.bar}",
-				},
-			},
-			map[string]interface{}{
-				"ami": "bar",
-				"security_groups": []interface{}{
-					"foo",
-					"bar",
-				},
-			},
-		},
-		{
-			map[string]interface{}{
-				"foo": map[string]interface{}{
-					"foo": []string{"${var.bar}"},
-				},
-			},
-			map[string]interface{}{
-				"foo": map[string]interface{}{
-					"foo": []string{"bar"},
-				},
-			},
-		},
-		{
-			map[string]interface{}{
-				"foo": "bar",
-				"bar": "hello${var.unknown}world",
-			},
-			map[string]interface{}{
-				"foo": "bar",
-			},
-		},
-		{
-			map[string]interface{}{
-				"foo": []string{"foo", "${var.unknown}", "bar"},
-			},
-			map[string]interface{}{},
-		},
-	}
-
-	for i, tc := range cases {
-		var input interface{} = tc.Input
-		if reflect.ValueOf(tc.Input).Kind() == reflect.String {
-			input = &tc.Input
-		}
-
-		if err := reflectwalk.Walk(input, w); err != nil {
-			t.Fatalf("err: %s", err)
-		}
-
-		if !reflect.DeepEqual(tc.Input, tc.Output) {
-			t.Fatalf("bad %d: %#v", i, tc.Input)
-		}
-	}
-}
-
-func TestVariableReplaceWalker_unknown(t *testing.T) {
-	cases := []struct {
-		Input  interface{}
-		Output interface{}
-		Keys   []string
-	}{
-		{
-			map[string]interface{}{
-				"foo": "bar",
-				"bar": "hello${var.unknown}world",
-			},
-			map[string]interface{}{
-				"foo": "bar",
-			},
-			[]string{"bar"},
-		},
-		{
-			map[string]interface{}{
-				"foo": []string{"foo", "${var.unknown}", "bar"},
-			},
-			map[string]interface{}{},
-			[]string{"foo"},
-		},
-		{
-			map[string]interface{}{
-				"foo": map[string]interface{}{
-					"bar": "${var.unknown}",
-				},
-			},
-			map[string]interface{}{
-				"foo": map[string]interface{}{},
-			},
-			[]string{"foo.bar"},
-		},
-	}
-
-	for i, tc := range cases {
-		var input interface{} = tc.Input
-		w := &variableReplaceWalker{
-			Values: map[string]string{
-				"var.unknown": UnknownVariableValue,
-			},
-		}
-
-		if reflect.ValueOf(tc.Input).Kind() == reflect.String {
-			input = &tc.Input
-		}
-
-		if err := reflectwalk.Walk(input, w); err != nil {
-			t.Fatalf("err: %s", err)
-		}
-
-		if !reflect.DeepEqual(tc.Input, tc.Output) {
-			t.Fatalf("bad %d: %#v", i, tc.Input)
-		}
-
-		if !reflect.DeepEqual(tc.Keys, w.UnknownKeys) {
-			t.Fatalf("bad: %#v", w.UnknownKeys)
-		}
-	}
-}
diff --git a/helper/multierror/error.go b/helper/multierror/error.go
index cddd2fc841..5bfe2b14d7 100644
--- a/helper/multierror/error.go
+++ b/helper/multierror/error.go
@@ -23,6 +23,10 @@ func (e *Error) Error() string {
 		len(e.Errors), strings.Join(points, "\n"))
 }
 
+func (e *Error) GoString() string {
+	return fmt.Sprintf("*%#v", *e)
+}
+
 // ErrorAppend is a helper function that will append more errors
 // onto a Error in order to create a larger multi-error. If the
 // original error is not a Error, it will be turned into one.
diff --git a/terraform/context.go b/terraform/context.go
index 1fa95ae0de..7674714ac0 100644
--- a/terraform/context.go
+++ b/terraform/context.go
@@ -30,6 +30,7 @@ type Context struct {
 	providers    map[string]ResourceProviderFactory
 	provisioners map[string]ResourceProvisionerFactory
 	variables    map[string]string
+	defaultVars  map[string]string
 
 	l     sync.Mutex    // Lock acquired during any task
 	parCh chan struct{} // Semaphore used to limit parallelism
@@ -72,6 +73,14 @@ func NewContext(opts *ContextOpts) *Context {
 	}
 	parCh := make(chan struct{}, par)
 
+	// Calculate all the default variables
+	defaultVars := make(map[string]string)
+	for _, v := range opts.Config.Variables {
+		for k, val := range v.DefaultsMap() {
+			defaultVars[k] = val
+		}
+	}
+
 	return &Context{
 		config:       opts.Config,
 		diff:         opts.Diff,
@@ -80,6 +89,7 @@ func NewContext(opts *ContextOpts) *Context {
 		providers:    opts.Providers,
 		provisioners: opts.Provisioners,
 		variables:    opts.Variables,
+		defaultVars:  defaultVars,
 
 		parCh: parCh,
 		sh:    sh,
@@ -296,8 +306,13 @@ func (c *Context) computeVars(raw *config.RawConfig) error {
 		return nil
 	}
 
-	// Go through each variable and find it
+	// Start building up the variables. First, defaults
 	vs := make(map[string]string)
+	for k, v := range c.defaultVars {
+		vs[k] = v
+	}
+
+	// Next, the actual computed variables
 	for n, rawV := range raw.Variables {
 		switch v := rawV.(type) {
 		case *config.ResourceVariable:
@@ -314,7 +329,19 @@ func (c *Context) computeVars(raw *config.RawConfig) error {
 
 			vs[n] = attr
 		case *config.UserVariable:
-			vs[n] = c.variables[v.Name]
+			val, ok := c.variables[v.Name]
+			if ok {
+				vs[n] = val
+				continue
+			}
+
+			// Look up if we have any variables with this prefix because
+			// those are map overrides. Include those.
+			for k, val := range c.variables {
+				if strings.HasPrefix(k, v.Name+".") {
+					vs["var."+k] = val
+				}
+			}
 		}
 	}
 
diff --git a/terraform/context_test.go b/terraform/context_test.go
index 40143f36d8..9d161ea6bf 100644
--- a/terraform/context_test.go
+++ b/terraform/context_test.go
@@ -1045,10 +1045,19 @@ func TestContextApply_vars(t *testing.T) {
 			"aws": testProviderFuncFixed(p),
 		},
 		Variables: map[string]string{
-			"foo": "bar",
+			"foo":            "us-west-2",
+			"amis.us-east-1": "override",
 		},
 	})
 
+	w, e := ctx.Validate()
+	if len(w) > 0 {
+		t.Fatalf("bad: %#v", w)
+	}
+	if len(e) > 0 {
+		t.Fatalf("bad: %s", e)
+	}
+
 	if _, err := ctx.Plan(nil); err != nil {
 		t.Fatalf("err: %s", err)
 	}
@@ -1159,10 +1168,6 @@ func TestContextPlan_computed(t *testing.T) {
 		t.Fatalf("err: %s", err)
 	}
 
-	if len(plan.Diff.Resources) < 2 {
-		t.Fatalf("bad: %#v", plan.Diff.Resources)
-	}
-
 	actual := strings.TrimSpace(plan.String())
 	expected := strings.TrimSpace(testTerraformPlanComputedStr)
 	if actual != expected {
diff --git a/terraform/semantics_test.go b/terraform/semantics_test.go
index 9d7d30ebc3..018f448b1f 100644
--- a/terraform/semantics_test.go
+++ b/terraform/semantics_test.go
@@ -18,4 +18,13 @@ func TestSMCUserVariables(t *testing.T) {
 	if len(errs) != 0 {
 		t.Fatalf("err: %#v", errs)
 	}
+
+	// Mapping element override
+	errs = smcUserVariables(c, map[string]string{
+		"foo":     "bar",
+		"map.foo": "baz",
+	})
+	if len(errs) != 0 {
+		t.Fatalf("err: %#v", errs)
+	}
 }
diff --git a/terraform/terraform_test.go b/terraform/terraform_test.go
index 635148d931..24e780f9e1 100644
--- a/terraform/terraform_test.go
+++ b/terraform/terraform_test.go
@@ -221,10 +221,13 @@ aws_instance.foo:
 const testTerraformApplyVarsStr = `
 aws_instance.bar:
   ID = foo
-  foo = bar
+  bar = foo
+  baz = override
+  foo = us-west-2
   type = aws_instance
 aws_instance.foo:
   ID = foo
+  bar = baz
   num = 2
   type = aws_instance
 `
diff --git a/terraform/test-fixtures/apply-compute/main.tf b/terraform/test-fixtures/apply-compute/main.tf
index 3999b59124..8fe55ed17a 100644
--- a/terraform/test-fixtures/apply-compute/main.tf
+++ b/terraform/test-fixtures/apply-compute/main.tf
@@ -1,3 +1,7 @@
+variable "value" {
+    default = ""
+}
+
 resource "aws_instance" "foo" {
     num = "2"
     compute = "dynamical"
diff --git a/terraform/test-fixtures/apply-vars/main.tf b/terraform/test-fixtures/apply-vars/main.tf
index f5c9c684ff..6645e532d0 100644
--- a/terraform/test-fixtures/apply-vars/main.tf
+++ b/terraform/test-fixtures/apply-vars/main.tf
@@ -1,7 +1,23 @@
+variable "amis" {
+    default = {
+        "us-east-1": "foo",
+        "us-west-2": "foo",
+    }
+}
+
+variable "bar" {
+    default = "baz"
+}
+
+variable "foo" {}
+
 resource "aws_instance" "foo" {
     num = "2"
+    bar = "${var.bar}"
 }
 
 resource "aws_instance" "bar" {
     foo = "${var.foo}"
+    bar = "${lookup(var.amis, var.foo)}"
+    baz = "${var.amis.us-east-1}"
 }
diff --git a/terraform/test-fixtures/smc-uservars/main.tf b/terraform/test-fixtures/smc-uservars/main.tf
index 7e2d10c110..cfa3293784 100644
--- a/terraform/test-fixtures/smc-uservars/main.tf
+++ b/terraform/test-fixtures/smc-uservars/main.tf
@@ -6,3 +6,10 @@ variable "foo" {
 variable "bar" {
     default = "baz"
 }
+
+# Mapping
+variable "map" {
+    default = {
+        "foo" = "bar";
+    }
+}