diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
index 6e5ec98df..9c076db2d 100644
--- a/config/interpolate_funcs.go
+++ b/config/interpolate_funcs.go
@@ -1,5 +1,10 @@
 package config
 
+import (
+	"fmt"
+	"strings"
+)
+
 // Funcs is the mapping of built-in functions for configuration.
 var Funcs map[string]InterpolationFunc
 
@@ -13,5 +18,18 @@ func init() {
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(
 	vs map[string]string, args ...string) (string, error) {
-	return "", nil
+	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 000000000..20f964c43
--- /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)
+		}
+	}
+}