XSLT is a rule-based language defined by the W3C for the transformation of XML-documents into XML-documents. XML-documents are essentially labelled rooted ordered trees or equivalently terms without arity restrictions. This motivates us to use term rewriting, a well-studied paradigm for the transformation of terms, to define and implement XSLT transformation. On the one hand term rewriting is conceptually simple and may be used as a formal definition of XSLT, on the other hand it can be used as a programming language and executed efficiently. We define a core fragment of XSLT in the ELAN language, developed in the PROTHEO group over recent years. The code generated by the ELAN compiler is efficient enough to use this definition directly to transform nontrivial documents.