The following technical report is available from http://aib.informatik.rwth-aachen.de/: Herbert Kuchen (ed.) WFLP 04 -- 13th International Workshop on Functional and (Constraint) Logic Programming 2004-05 This report contains the proceedings of the 13th International Workshop on Functional and (Constraint) Logic Programming (WFLP 2004), which was held on June 1, 2004 as part of the Federated Conference on Rewriting, Deduction, and Programming (RDP 2004) in Aachen, Germany.