diff --git a/src/evaluator/env.rs b/src/evaluator/env.rs
index 685acb9..b9f63dd 100644
--- a/src/evaluator/env.rs
+++ b/src/evaluator/env.rs
@@ -1,6 +1,6 @@
 use std::{borrow::Borrow, cell::RefCell, collections::HashMap, rc::Rc};
 
-use super::{eval_ast_node, Error, Expression};
+use super::{eval_ast_node, Error, Expression, FALSE, TRUE};
 use crate::parser::Node;
 
 pub type RawEnvironment = HashMap<String, Rc<Expression>>;
@@ -127,6 +127,20 @@ pub fn core_environment() -> Rc<Environment> {
                 Ok(Rc::new(Expression::HashMap(res)))
             }),
         ),
+        (
+            "not",
+            Expression::NativeFunc(|args| {
+                if args.len() != 1 {
+                    Err(Error::MismatchedArgCount(1, args.len()))?
+                }
+
+                if FALSE.with(|inner| &args[0] == inner) {
+                    return Ok(TRUE.with(|inner| inner.clone()));
+                }
+
+                return Ok(FALSE.with(|inner| inner.clone()));
+            }),
+        ),
         // Branching
         (
             "if",
diff --git a/src/evaluator/mod.rs b/src/evaluator/mod.rs
index 33d146c..a50d96d 100644
--- a/src/evaluator/mod.rs
+++ b/src/evaluator/mod.rs
@@ -219,6 +219,10 @@ mod test {
     #[case("((fn* (a) a) 3)", "3")]
     #[case("((fn* (a) (+ a 2)) 1)", "3")]
     #[case("((fn* (a b) (+ a b)) 1 2)", "3")]
+    #[case("(not false)", "true")]
+    #[case("(not true)", "false")]
+    #[case("(not nil)", "false")]
+    #[case("(not 1)", "false")]
     fn test_evaluator(#[case] input: &str, #[case] expected: &str) {
         let env = core_environment();
         let tokens = lexer::read(input).unwrap();