This book constitutes the thoroughly refereed post-proceedings of the 6th International Workshop on Multi-Agent-Based Simulation, MABS 2005, held in Utrecht, The Netherlands in July 2005 as an associated event of AAMAS 2005, the main international conference on autonomous agents and multi-agent systems. The 12 revised full papers presented together with 1 invited talk were carefully reviewed. The papers are organized in topical sections on coalition emergence, theories and models, applications, and environments.