diff --git a/eval.go b/eval.go
index 40573fbdf..8012de41a 100644
--- a/eval.go
+++ b/eval.go
@@ -282,6 +282,27 @@ func (n methodNode) eval(env interface{}) (interface{}, error) {
 	return call(n.property.value, method, n.arguments, env)
 }
 
+func (n builtinNode) eval(env interface{}) (interface{}, error) {
+	if len(n.arguments) == 0 {
+		return nil, fmt.Errorf("missing argument to %v", n.name)
+	}
+	if len(n.arguments) > 1 {
+		return nil, fmt.Errorf("too many arguments to %v: %v", n.name, n)
+	}
+
+	a, err := Run(n.arguments[0], env)
+	if err != nil {
+		return nil, err
+	}
+
+	switch n.name {
+	case "len":
+		return count(n.arguments[0], a)
+	}
+
+	return nil, fmt.Errorf("unknown %q builtin", n.name)
+}
+
 func (n functionNode) eval(env interface{}) (interface{}, error) {
 	fn, err := extract(env, n.name)
 	if err != nil {
diff --git a/eval_test.go b/eval_test.go
index fc1ef8fee..73b4d492a 100644
--- a/eval_test.go
+++ b/eval_test.go
@@ -149,6 +149,16 @@ var evalTests = []evalTest{
 		map[string]interface{}{"foo": []rune{'a', 'b', 'c'}},
 		'c',
 	},
+	{
+		"len(foo) == 3",
+		map[string]interface{}{"foo": []rune{'a', 'b', 'c'}},
+		true,
+	},
+	{
+		`len(foo) == 6`,
+		map[string]string{"foo": "foobar"},
+		true,
+	},
 	{
 		"[1, 2, 3][2/2]",
 		nil,
@@ -359,6 +369,21 @@ var evalErrorTests = []evalErrorTest{
 		nil,
 		"operator in not defined on string",
 	},
+	{
+		"len(1)",
+		nil,
+		"invalid argument 1 (type float64) for len",
+	},
+	{
+		"len(foo, bar)",
+		map[string]interface{}{"foo": nil, "bar": nil},
+		"too many arguments to len: len(foo, bar)",
+	},
+	{
+		"len()",
+		nil,
+		"missing argument to len",
+	},
 }
 
 func TestEval(t *testing.T) {
diff --git a/node.go b/node.go
index e317ccf74..5abef3da2 100644
--- a/node.go
+++ b/node.go
@@ -1,6 +1,6 @@
 package expr
 
-// Node items of abstract syntax tree.
+// Node represents items of abstract syntax tree.
 type Node interface{}
 
 type nilNode struct{}
@@ -47,6 +47,11 @@ type methodNode struct {
 	arguments []Node
 }
 
+type builtinNode struct {
+	name      string
+	arguments []Node
+}
+
 type functionNode struct {
 	name      string
 	arguments []Node
diff --git a/parser.go b/parser.go
index 9524dd9ab..b3c72da5d 100644
--- a/parser.go
+++ b/parser.go
@@ -52,6 +52,10 @@ var binaryOperators = map[string]info{
 	"**":      {200, right},
 }
 
+var builtins = map[string]bool{
+	"len": true,
+}
+
 type parser struct {
 	input    string
 	tokens   []token
@@ -284,16 +288,24 @@ func (p *parser) parsePrimaryExpression() (Node, error) {
 			return nilNode{}, nil
 		default:
 			if p.current.is(punctuation, "(") {
-				if p.options.funcs != nil {
-					if _, ok := p.options.funcs[token.value]; !ok {
-						return nil, p.errorf("unknown func %v", token.value)
+				if _, ok := builtins[token.value]; ok {
+					arguments, err := p.parseArguments()
+					if err != nil {
+						return nil, err
 					}
+					node = builtinNode{name: token.value, arguments: arguments}
+				} else {
+					if p.options.funcs != nil {
+						if _, ok := p.options.funcs[token.value]; !ok {
+							return nil, p.errorf("unknown func %v", token.value)
+						}
+					}
+					arguments, err := p.parseArguments()
+					if err != nil {
+						return nil, err
+					}
+					node = functionNode{name: token.value, arguments: arguments}
 				}
-				arguments, err := p.parseArguments()
-				if err != nil {
-					return nil, err
-				}
-				node = functionNode{name: token.value, arguments: arguments}
 			} else {
 				if p.options.names != nil {
 					if _, ok := p.options.names[token.value]; !ok {
diff --git a/parser_test.go b/parser_test.go
index 1c05e7976..b7f30f79a 100644
--- a/parser_test.go
+++ b/parser_test.go
@@ -121,6 +121,10 @@ var parseTests = []parseTest{
 		"{foo:1}.bar",
 		propertyNode{mapNode{[]pairNode{{identifierNode{"foo"}, numberNode{1}}}}, identifierNode{"bar"}},
 	},
+	{
+		"len(foo)",
+		builtinNode{"len", []Node{nameNode{"foo"}}},
+	},
 }
 
 type parseErrorTest struct {
diff --git a/print.go b/print.go
index 99c15da4d..992aac45b 100644
--- a/print.go
+++ b/print.go
@@ -63,6 +63,17 @@ func (n methodNode) String() string {
 	return s + ")"
 }
 
+func (n builtinNode) String() string {
+	s := fmt.Sprintf("%v(", n.name)
+	for i, a := range n.arguments {
+		if i != 0 {
+			s += ", "
+		}
+		s += fmt.Sprintf("%v", a)
+	}
+	return s + ")"
+}
+
 func (n functionNode) String() string {
 	s := fmt.Sprintf("%v(", n.name)
 	for i, a := range n.arguments {
diff --git a/utils.go b/utils.go
index b4bb8e05a..b5ea03f37 100644
--- a/utils.go
+++ b/utils.go
@@ -120,6 +120,21 @@ func contains(needle interface{}, array interface{}) (bool, error) {
 	return false, nil
 }
 
+func count(node Node, array interface{}) (float64, error) {
+	if array != nil {
+		value := reflect.ValueOf(array)
+		switch reflect.TypeOf(array).Kind() {
+		case reflect.Array, reflect.Slice:
+			return float64(value.Len()), nil
+		case reflect.String:
+			return float64(value.Len()), nil
+		}
+		return 0, fmt.Errorf("invalid argument %v (type %T) for len", node, array)
+	}
+
+	return 0, nil
+}
+
 func call(name string, fn interface{}, arguments []Node, env interface{}) (interface{}, error) {
 	in := make([]reflect.Value, 0)
 	for _, arg := range arguments {