This book constitutes the refereed proceedings of the 23rd IFIP WG 6.1 International Conference on Formal Techniques for Networked and Distributed Systems, FORTE 2003, held in Berlin, Germany in September/October 2003.The 24 revised full papers presented together with 3 invited papers were carefully reviewed and selected from 55 submissions. The papers are organized in topical sections on application of formal description techniques (FDTs), verification, timed automata, verification of security protocols, testing, and FDT-based design.