We describe a set of tools that detect when World Wide Web pages have been modified and present the modifications visually to the user through marked up HTML. The tools consist of three components: w3newer, which detects changes to pages; snapshot, which permits a user to store a copy of an arbitrary Web page and to compare any subsequent version of a page with the saved version; and HtmlDiff which marks up HTML text to indicate how it has changed from a previous version. We refer to the tools collectively as the AT&T Internet Difference Engine (AIDE). The paper discusses several aspects of AIDE, With an emphasis on systems issues such as scalability, security and error conditions.