This book constitutes the thoroughly refereed post-proceedings of the two International Workshops on Agent Communication, AC 2005 and AC 2006, held in Utrecht, Netherlands in July 2005, and in Hakodate, Japan in May 2006 as associated events of AAMAS 2005/2006, the main international conference on autonomous agents and multi-agent systems. The 20 revised full papers presented were carefully reviewed and selected - they represent the best papers from the Agent Communication workshops of 2005 and 2006, enriched with a few revised agent communication papers from the AAMAS conference. The papers are organized in topical sections on semantics of agent communication, commitments in agent communication, protocols and strategies, as well as reliability and overhearing.