The TXL source transformation language. TXL is a special-purpose programming language designed for creating, manipulating and rapidly prototyping language descriptions, tools and applications. TXL is designed to allow explicit programmer control over the interpretation, application, order and backtracking of both parsing and rewriting rules. Using first order functional programming at the higher level and term rewriting at the lower level, TXL provides for flexible programming of traversals, guards, scope of application and parameterized context. This flexibility has allowed TXL users to express and experiment with both new ideas in parsing, such as robust, island and agile parsing, and new paradigms in rewriting, such as XML mark-up, rewriting strategies and contextualized rules, without any change to TXL itself. This paper outlines the history, evolution and concepts of TXL with emphasis on its distinctive style and philosophy, and gives examples of its use in expressing and applying recent new paradigms in language processing.