This volume contains selected papers from WADT 2004, the 17th International Workshop on Algebraic Development Techniques. Like its predecessors, WADT 2004 focussed on the algebraic approach to the speci?cation and development of systems, an area that was born around the algebraic speci?cation of abstract data types and encompasses today the formal design of software systems, new speci?cation frameworks and a wide range of application areas. WADT 2004 took place at the Technical University of Catalonia (UPC), Barcelona, Spain, on 27-29 March 2004, and was organized by Fernando Orejas and Jordi Cortadella. ' The program consisted of invited talks by Luis Caires (Universidade Nova de Lisboa, Portugal) and Reiko Heckel (University of Paderborn, Germany), and 33 presentations describing ongoing research on main topics of the workshop: formal methods for system development, speci?cation languages and methods, systems and techniques for reasoning about speci?cations, speci?cation devel- ment systems, methods and techniques for concurrent, distributed and mobile systems, and algebraic and co-algebraic foundations.