diff --git a/src/evaluator/builtins.rs b/src/evaluator/builtins.rs
new file mode 100644
index 0000000..888176b
--- /dev/null
+++ b/src/evaluator/builtins.rs
@@ -0,0 +1,28 @@
+use std::{cell::LazyCell, collections::HashMap};
+
+use super::{BuiltinFunction, Object};
+
+pub const BUILTINS: LazyCell<HashMap<&'static str, Object>> = LazyCell::new(|| {
+    let mut map = HashMap::new();
+
+    map.insert(
+        "len",
+        Object::Builtin(BuiltinFunction {
+            func: Box::new(|args: Vec<Object>| {
+                if args.len() != 1 {
+                    return Object::Error(format!(
+                        "wrong number of arguments. got={}, want=1",
+                        args.len()
+                    ));
+                }
+
+                match &args[0] {
+                    Object::String(s) => Object::Integer(s.len() as i64),
+                    v => Object::Error(format!("argument to `len` not supported, got {}", v)),
+                }
+            }),
+        }),
+    );
+
+    map
+});
diff --git a/src/evaluator/mod.rs b/src/evaluator/mod.rs
index 28fe286..9d7e055 100644
--- a/src/evaluator/mod.rs
+++ b/src/evaluator/mod.rs
@@ -4,10 +4,11 @@ use {
     std::{
         cell::RefCell,
         collections::HashMap,
-        fmt::{Display, Formatter, Result as FmtResult, Write},
+        fmt::{self, Display, Formatter, Result as FmtResult, Write},
         rc::Rc,
     },
 };
+pub mod builtins;
 pub mod tree_walker;
 
 pub trait Evaluator {
@@ -59,13 +60,10 @@ pub enum Object {
     ReturnValue(Box<Object>),
     Error(String),
     Function(Function),
+    Builtin(BuiltinFunction),
     Null,
 }
 
-const NULL: Object = Object::Null;
-const TRUE: Object = Object::Boolean(true);
-const FALSE: Object = Object::Boolean(false);
-
 impl Object {
     pub fn inspect(&self) -> String {
         match self {
@@ -75,6 +73,7 @@ impl Object {
             Object::ReturnValue(ret) => ret.inspect(),
             Object::Error(s) => s.to_string(),
             Object::Null => "NULL".into(),
+            Object::Builtin(_) => "builtin function".to_string(),
             Object::Function(s) => {
                 let mut out = String::new();
 
@@ -101,6 +100,7 @@ impl Display for Object {
             Object::Error(_) => "ERROR",
             Object::Function(_) => "FUNCTION",
             Object::Null => "NULL",
+            Object::Builtin(_) => "BUILTIN",
         })
     }
 }
@@ -112,15 +112,39 @@ pub struct Function {
     env: Rc<RefCell<Environment>>,
 }
 
+type Builtin = fn(Vec<Object>) -> Object;
+
+#[derive(Clone)]
+pub struct BuiltinFunction {
+    func: Box<Builtin>,
+}
+
+impl fmt::Debug for BuiltinFunction {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        f.debug_struct("BuiltinFunction")
+            .field("func", &"builtin")
+            .finish()
+    }
+}
+
+impl PartialEq for BuiltinFunction {
+    fn eq(&self, _: &Self) -> bool {
+        false
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::{assert_matches::assert_matches, cell::RefCell, rc::Rc};
 
     use crate::{
-        evaluator::{tree_walker::TreeWalker, Environment, Evaluator, Object, FALSE, NULL, TRUE},
+        evaluator::{tree_walker::TreeWalker, Environment, Evaluator, Object},
         lexer::Lexer,
         parser::{ast::Node, Parser},
     };
+    const TRUE: Object = Object::Boolean(true);
+    const FALSE: Object = Object::Boolean(false);
+    const NULL: Object = Object::Null;
 
     fn run_test_cases(test_cases: &[(&str, Option<Object>)]) {
         for test in test_cases.iter() {
@@ -411,4 +435,29 @@ mod tests {
 
         run_test_cases(&test_cases);
     }
+
+    #[test]
+    fn builtin_function() {
+        // This test case allocates memory for `foobar=9999` over and over
+        // even though it is never used.
+        let test_cases = [
+            ("len(\"\")", Some(Object::Integer(0))),
+            ("len(\"four\")", Some(Object::Integer(4))),
+            ("len(\"hello world\")", Some(Object::Integer(11))),
+            (
+                "len(1)",
+                Some(Object::Error(
+                    "argument to `len` not supported, got INTEGER".to_string(),
+                )),
+            ),
+            (
+                "len(\"one\", \"two\")",
+                Some(Object::Error(
+                    "wrong number of arguments. got=2, want=1".to_string(),
+                )),
+            ),
+        ];
+
+        run_test_cases(&test_cases);
+    }
 }
diff --git a/src/evaluator/tree_walker.rs b/src/evaluator/tree_walker.rs
index 377c7b5..2b97f0f 100644
--- a/src/evaluator/tree_walker.rs
+++ b/src/evaluator/tree_walker.rs
@@ -4,7 +4,7 @@ use std::{cell::RefCell, rc::Rc};
 // It's just constantly unwrapping enums from one place and rewrapping it to some other enum(or even the same enum) and returning it
 // The error handling story is pretty bad too
 use crate::{
-    evaluator::{Evaluator, Object, FALSE, NULL, TRUE},
+    evaluator::{Evaluator, Object},
     lexer::TokenType,
     parser::ast::{
         BlockStatement, Expression, ExpressionStatement, Identifier, LetStatement, Node, Program,
@@ -12,7 +12,7 @@ use crate::{
     },
 };
 
-use super::{Environment, Function};
+use super::{builtins::BUILTINS, Environment, Function};
 
 pub struct TreeWalker;
 
@@ -84,7 +84,7 @@ impl Evaluator for TreeWalker {
                     } else if let Some(alternative) = ie.alternative {
                         self.eval(Node::Statement(Statement::BlockStatement(alternative)), env)
                     } else {
-                        Some(NULL)
+                        Some(Object::Null)
                     }
                 }
             },
@@ -195,10 +195,10 @@ impl TreeWalker {
 
     fn eval_bang_operator_expression(&self, expr: Object) -> Object {
         match expr {
-            TRUE => FALSE,
-            FALSE => TRUE,
-            NULL => TRUE,
-            _ => FALSE,
+            Object::Boolean(true) => Object::Boolean(false),
+            Object::Boolean(false) => Object::Boolean(true),
+            Object::Null => Object::Boolean(true),
+            _ => Object::Boolean(false),
         }
     }
 
@@ -211,19 +211,25 @@ impl TreeWalker {
 
     fn is_truthy(&self, obj: &Object) -> bool {
         match *obj {
-            NULL => false,
-            TRUE => true,
-            FALSE => false,
+            Object::Null => false,
+            Object::Boolean(true) => true,
+            Object::Boolean(false) => false,
             _ => true,
         }
     }
 
     fn eval_identifier(&self, node: Identifier, env: Rc<RefCell<Environment>>) -> Option<Object> {
         let env = env.borrow();
-        env.get(&node.to_string()).or(Some(Object::Error(format!(
-            "identifier not found: {}",
-            node.to_string()
-        ))))
+
+        if let Some(v) = env.get(&node.to_string()) {
+            return Some(v.clone());
+        }
+
+        if let Some(v) = BUILTINS.get(&node.to_string().as_str()) {
+            return Some(v.clone());
+        }
+
+        Some(Object::Error(format!("identifier not found: {}", node)))
     }
 
     fn eval_expression(
@@ -247,18 +253,22 @@ impl TreeWalker {
     }
 
     fn apply_function(&self, function: Object, args: Vec<Object>) -> Option<Object> {
+        if let Object::Builtin(ref func) = function {
+            return Some((func.func)(args));
+        }
+
         let function = match function {
             Object::Function(f) => f,
             Object::Error(e) => {
-                return Some(Object::Error(format!("not a function: {}", e.to_string())));
+                return Some(Object::Error(format!("not a function: {}", e)));
             }
-            v => return Some(Object::Error(format!("not a function: {}", v.to_string()))),
+            v => return Some(Object::Error(format!("not a function: {}", v))),
         };
 
         let mut enclosed_env = Environment::new_enclosed(function.env);
         for (i, parameter) in function.parameters.iter().enumerate() {
             if args.len() <= i {
-                return Some(Object::Error(format!("incorrect number of arguments")));
+                return Some(Object::Error("incorrect number of arguments".to_owned()));
             }
 
             enclosed_env.set(parameter.value.clone(), args[i].clone());
diff --git a/src/main.rs b/src/main.rs
index 2621f72..f51a8b9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,4 @@
+#![feature(lazy_cell)]
 #![feature(assert_matches)]
 #[macro_use]
 extern crate lazy_static;