This book presents the refereed proceedings of the 15th International Symposium on Formal Methods, FM 2008, held in Turku, Finland in May 2008. The 23 revised full papers presented together with 4 invited contributions and extended abstracts of 5 invited industrial presentations were carefully reviewed and selected from 106 submissions. The papers are organized in topical sections on programming language analysis, verification, real-time and concurrency, grand chellenge problems, fm practice, runtime monitoring and analysis, communication, constraint analysis, and design.