This volume contains the proceedings of the 15th International Conference on Concurrency Theory (CONCUR 2004) held in the Royal Society, London, UK, from the 31st August to the 3rd September, 2004. ThepurposeoftheCONCURconferencesistobringtogetherresearchers, - velopersandstudentsinordertoadvancethetheoryofconcurrencyandpromote its applications. Interest in this topic is continually growing, as a consequence of theimportanceandubiquityofconcurrentsystemsandtheirapplications, andof the scienti?c relevance of their foundations. The scope covers all areas of sem- tics, logics, and veri?cation techniques for concurrent systems. Topics include concurrency-related aspects of: models of computation, semantic domains, p- cess algebras, Petri nets, event structures, real-time systems, hybrid systems, - cidability, model-checking, veri?cation techniques, re?nement techniques, term and graph rewriting, distributed programming, logic constraint programming, object-oriented programming, typing systems and algorithms, case studies, tools and environments for programming and veri?cation. This volume starts with four invited papers from Sriram Rajamani, Steve Brookes, Bengt Jonsson and Peter O'Hearn. The remaining 29 papers were - lected by the program committee from 134 submissions, a record number of submissions to CONCUR. The standard was extremely high and the selection di?cult. Each submission received at least three reports, reviewed by the p- gram committee members or their subreferees. Once the initial reviews were available, we had 16 days for paper selection and con?ict resolution. We would like to thank all members of the CONCUR 2004 Program Committee for their excellent work throughout the intensive selection process, together with many subreferees who assisted us in the evaluation of the submitted papers.