Book excerpt: 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 20 revised full papers cover semantics of agent communication, commitments in agent communication, protocols and strategies, as well as reliability and overhearing.