The 16th International Workshop on Implementation and Application of Fu- tional Languages (IFL 2004) was held in Lub ¨ eck, Germany, September 8–10, 2004. It was jointly organized by the Institute of Computer Science and Applied Mathematics of the University of Kiel and the Institute of Software Technology and Programming Languages of the University of Lub ¨ eck. IFL 2004 was the sixteenth event in the annual series of IFL workshops. The aim of the workshop series is to bring together researchers actively engaged in the implementation and application of functional and function-based progr- ming languages. It provides an open forum for researchers who wish to present and discuss new ideas and concepts, work in progress, preliminary results, etc., related primarily, but not exclusively, to the implementation and application of functional languages. Topics of interest cover a wide range from theoretical - pects over language design and implementation towards applications and tool support. Previous IFL workshops were held in the United Kingdom (Southampton, Norwich, London, St Andrews, and Edinburgh), in the Netherlands (Nijmegen and Lochem), in Germany (Aachen and Bonn), in Sweden (B? astad and Sto- holm), and in Spain (Madrid). In 2005, the 17th International Workshop on - plementation and Application of Functional Languages will be held in Dublin, Ireland.