diff --git a/README.md b/README.md
index 509583c..0bd81c8 100644
--- a/README.md
+++ b/README.md
@@ -162,6 +162,7 @@ quote | `(quote (1 2 3))` | `(1 2 3)` | Return the argument without evaluation.
 
 Name  | Example | Example result | Description
 ----- | ------- | -------------- | -----------
+debug | `(debug)` | `true` |  Toggle debug output.
 doc   | `(doc +)` | `"Return the sum of all arguments."` | Show description of a built-in function.
 loop  | `(loop (fn (a) (do (print a) (coinflip))) "hello ")` | `hello hello hello false` | Call the given function repeatedly in a loop until it returns false.
 meta  | `(meta (env) "name")` | `"root/user"` | Read meta information of an entity.
diff --git a/src/Evaller.php b/src/Evaller.php
index e6bddb4..ff75bf9 100644
--- a/src/Evaller.php
+++ b/src/Evaller.php
@@ -294,6 +294,11 @@ class Evaller
         }
     }
 
+    public function getDebug(): bool
+    {
+        return $this->debug;
+    }
+
     public function setDebug(bool $val): void
     {
         $this->debug = $val;
diff --git a/src/LispFactory.php b/src/LispFactory.php
index f77a73a..d0308fb 100644
--- a/src/LispFactory.php
+++ b/src/LispFactory.php
@@ -18,6 +18,14 @@ class LispFactory
         $env->set('__DIR__', null);
 
         // Register core functions
+        $env->set('debug', new CoreFunc('debug', 'Toggle debug mode.', 0, 0,
+            function () use ($eval) {
+                $val = !$eval->getDebug();
+                $eval->setDebug($val);
+                return $val;
+            }
+        ));
+
         $env->set('doc', new CoreFunc('doc', 'Get documentation for a function.', 1, 1,
             function ($a) {
                 if ($a instanceof Func) {