Since version 2.3, XWiki integrates document annotations in its collaborative environment (http://extensions.xwiki.org/xwiki/bin/view/Extension/Annotations+Application). This talk will focus on the implementation of the annotation feature, with the challenges that an eXtensible wiki implies: handling user collaboration on documents and annotations, annotating dynamically generated content, flexible annotations structure to allow customization. As a default XWiki feature, it allows users to add notes on all types of content, plain unstructured documents or structured documents. It is also used in the Scribo project (http://www.scribo.ws) to manipulate automatic semantic annotations on wiki documents and collaboration around them. Since a demo is worth a thousand words, the presentation will center upon showing all these at work.