This book constitutes the thoroughly refereed post-conference proceedings of the 6th International Workshop on Programming Multi-Agent Systems, ProMAS 2008, held in Estoril, Portugal, in May 2008 as an associated event of AAMAS 2008, the 7th International Joint Conference on Autonomous Agents and Multiagent Systems. The 12 revised full papers presented together with 1 invited lecture address current issues in the areas of agent platforms, environment and interaction, agent programming languages, and analysis of MAS. The papers are organized in topical sections on agent programming languages, multi-agent systems frameworks, as well as agent environments and tools. The volume is concluded with a selection of 6 short papers related to the Agent Contest 2008 that has been organized since 2006 in conjunction with ProMAS.