 <p>I'm surprised you are not using the TYPO3's documentation...</p> <p>First hit with full description and samples: <a href="http://wiki.typo3.org/XCLASS" rel="nofollow">http://wiki.typo3.org/XCLASS</a></p> <blockquote> <p>XCLASS'ing is a mechanism in TYPO3 CMS to extend or overwrite classes or methods of other extensions or of core code with own code...</p> </blockquote>