This book constitutes the revised selected papers from the 13 European Conference on Multi-Agent Systems, EUMAS 2015, and the Third International Conference on Agreement Technologies, AT 2015, held in Athens, Greece, in December 2015. The 36 papers presented in this volume were carefully reviewed and selected from 65 submissions. They are organized in topical sections named: coordination and planning; learning and optimization, argumentation and negotiation; norms, trust, and reputation; agent-based simulation and agent programming.