diff --git a/config/interpolate.go b/config/interpolate.go
index dc4f39fd16..ab2cd26a40 100644
--- a/config/interpolate.go
+++ b/config/interpolate.go
@@ -2,10 +2,15 @@ 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.
 //
@@ -17,6 +22,10 @@ type Interpolation interface {
 	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
@@ -25,6 +34,15 @@ 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 {
@@ -69,6 +87,33 @@ type UserMapVariable struct {
 // 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++ {
+			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 {
@@ -99,6 +144,43 @@ func NewInterpolatedVariable(v string) (InterpolatedVariable, error) {
 	}
 }
 
+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
 }
@@ -166,6 +248,10 @@ func (v *UserVariable) FullKey() string {
 	return v.key
 }
 
+func (v *UserVariable) GoString() string {
+	return fmt.Sprintf("*%#v", *v)
+}
+
 func NewUserMapVariable(key string) (*UserMapVariable, error) {
 	name := key[len("var."):]
 	idx := strings.Index(name, ".")
diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
new file mode 100644
index 0000000000..6e5ec98dfb
--- /dev/null
+++ b/config/interpolate_funcs.go
@@ -0,0 +1,17 @@
+package config
+
+// 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) {
+	return "", nil
+}
diff --git a/config/interpolate_test.go b/config/interpolate_test.go
index 3e399bf655..87c2be0ca6 100644
--- a/config/interpolate_test.go
+++ b/config/interpolate_test.go
@@ -2,6 +2,7 @@ package config
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -28,6 +29,25 @@ func TestNewInterpolation(t *testing.T) {
 			},
 			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 {
@@ -35,6 +55,13 @@ func TestNewInterpolation(t *testing.T) {
 		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)
 		}
@@ -106,6 +133,55 @@ func TestNewUserVariable(t *testing.T) {
 	}
 }
 
+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)
 }