diff --git a/README.md b/README.md
index 3331bfd..2507c0f 100644
--- a/README.md
+++ b/README.md
@@ -163,7 +163,8 @@ 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.
+doc   | `(doc +)` | `"Return the sum of all arguments."` | Show the documentation string for a function.
+doc   | `(doc myfn "Documentation string.")` | `"Documentation string."` | Set the documentation string for a 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.
 read  | `(read "(+ 1 2 3)")` | `(+ 1 2 3)` | Read a string as code and return the expression.
diff --git a/mad/fact.mad b/mad/fact.mad
index 7f45a63..ae19d4c 100644
--- a/mad/fact.mad
+++ b/mad/fact.mad
@@ -5,3 +5,7 @@
 
 ;; Apply version
 (def applyFact (fn (n) (if (< n 2) 1 (apply * (range 1 (inc n))))))
+
+;; Add documentation
+(doc recFact "Calculate the factor of n recursively.")
+(doc applyFact "Calculate the factor of n iteratively using apply.")
diff --git a/mad/fib.mad b/mad/fib.mad
index 5318a14..bbb0416 100644
--- a/mad/fib.mad
+++ b/mad/fib.mad
@@ -10,3 +10,7 @@
 (def fibListRec (fn (n l) (if (< (len l) n) (fibListRec n (push l (sumOfLast l))) l)))
 (def fibList (fn (n) (fibListRec n [0 1])))
 (def fastFib (fn (n) (sumOfLast (fibList n))))
+
+;; Add documentation
+(doc slowFib "Return the nth number from the Fibonacci sequence recursively.")
+(doc fastFib "Return the nth number from the Fibonacci sequence iteratively.")
diff --git a/src/Func.php b/src/Func.php
index e8e9fe8..1049d1e 100644
--- a/src/Func.php
+++ b/src/Func.php
@@ -24,5 +24,10 @@ abstract class Func
         return $this->doc;
     }
 
+    public function setDoc(?string $val): void
+    {
+        $this->doc = $val;
+    }
+
     abstract public function call(array $args);
 }
diff --git a/src/Lib/Core.php b/src/Lib/Core.php
index 94d3349..2307184 100644
--- a/src/Lib/Core.php
+++ b/src/Lib/Core.php
@@ -43,13 +43,14 @@ class Core implements ILib
             }
         ));
 
-        $env->set('doc', new CoreFunc('doc', 'Get documentation for a function.', 1, 1,
-            function ($a) {
-                if ($a instanceof Func) {
+        $env->set('doc', new CoreFunc('doc', 'Get or set documentation for a function.', 1, 2,
+            function (Func $a, ?string $str = null) {
+                if (func_num_args() == 1) {
                     return $a->getDoc();
+                } else {
+                    $a->setDoc($str);
+                    return $str;
                 }
-
-                throw new MadLispException('first argument to doc is not function');
             }
         ));