This book constitutes the refereed proceedings of the 11th European PVM/MPI users' Group Meeting held in Budapest, Hungary, in September 2004. The 50 revised papers presented together with abstracts of 10 invited contributions were carefully reviewed and selected from numerous submissions. The papers are organized in topical sections on MPI/PVM extensions and improvements, algorithms, applications, tools and environments, cluster and grid computing, performance, and parallel numerical simulation.