diff --git a/README.md b/README.md
index 7e1f10e..7d72281 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@ Better Markdown Parser in PHP
 
 ### Installation
 
-Include `Parsedown.php` or install [the composer package](https://packagist.org/packages/erusev/parsedown).
+Include `Parsedown.php` or install [the composer package](https://packagist.org/packages/erusev/parsedown). Parsedown requires PHP 5.3+
 
 ### Example
 
diff --git a/composer.json b/composer.json
index 1439b82..28145af 100644
--- a/composer.json
+++ b/composer.json
@@ -12,7 +12,10 @@
             "homepage": "http://erusev.com"
         }
     ],
+    "require": {
+        "php": ">=5.3.0"
+    },
     "autoload": {
         "psr-0": {"Parsedown": ""}
     }
-}
\ No newline at end of file
+}