This book constitutes the proceedings of the 12th European Conference on Multi-Agent Systems, EUMAS 2014, held in Prague, Czech Republic, in December 2014. The 21 full papers and 8 short papers presented in this volume were carefully reviewed and selected from 79 submissions. They are organized in topical sections named: agent-based models, trust and reputation; coordination, coalitions and teamwork; logic and formal approaches; theories in practice and real-world problems; decision making, conflicts and agreements.