XML is now a dominant standard for storing and exchanging information. One very important activity is the transformation of XML documents into other formats, via the transformation language XSLT. XSLT provides a powerful way to perform document conversion and exchange, avoiding reliance on application speci.c solutions. However, XSLT is a complex language and the current level of support for debugging tools is poor. Many tools that do exist are mainly an extension of conventional techniques for imperative programs and not well-suited to the task. In this paper, we present CodeX, a debugger for XSLT and propose three debugging techniques which are particularly suited to the language We aim to offer XSLT users a tool which is beneficial in finding errors, as well as facilitating a better understanding of the XML transformation process.