This book constitutes the refereed proceedings of the 14th International Conference on Concurrency Theory, CONCUR 2003, held in Marseille, France in September 2003. The 29 revised full papers presented together with 4 invited papers were carefully reviewed and selected from 107 submissions. The papers are organized in topical sections on partial orders and asynchronous systems, process algebras, games, infinite systems, probabilistic automata, model checking, model checking and HMSC, security, mobility, compositional methods and real time, and probabilistic models.