This volume contains the proceedings of the 14th International Conference on ConcurrencyTheory(CONCUR2003)heldinMarseille, France, September3-5, 2003. The conference was hosted by the UniversitedeProvenceandtheLa- ratoire d'Informatique Fondamentale de Marseille (LIF). The purpose of the CONCUR conferences is to bring together researchers, developers, and students in order to advance the theory of concurrency, and promote its applications. Interest in this topic is continuously growing, as a consequence of the importance and ubiquity of concurrent systems and their applications, and of the scienti?c relevance of their fundations. The scope of the conference covers all areas of semantics, logics, and veri?cation techniques for concurrent systems. Topics include concurrency-related aspects of: models of computation and semantic domains, process algebras, Petri nets, event struc- res, real-time systems, hybrid systems, decidability, model-checking, veri?cation and re?nement techniques, term and graph rewriting, distributed programming, logic constraint programming, object-oriented programming, types systems and algorithms, case studies, and tools and environments for programming and - ri?cation. Of the 107 papers submitted this year, 29 were accepted for presentation. Four invited talks were given at the conference: on Distributed Monitoring of Concurrent and Asynchronous Systems by Albert Beneveniste, on Quantitative Veri?cation via the MU-Calculus by Luca De Alfaro, on Input-Output Au- mata: Basic, Timed, Hybrid, Probabilistic, Dynamic, . . . by Nancy Lynch, and on Composition of Cryptographic Protocols in a Probabilistic Polynomial-Time Process Calculus by Andre Scedrov."