This book constitutes the proceedings of the 12th German Conference on Multiagent System Technologies, MATES 2014, held in Stuttgart, Germany, in September 2014. The 9 full papers and 7 short papers included in this volume were carefully reviewed and selected from 31 submissions. The book also contains 2 invited talks. The papers are organized in topical sections named: mechanisms, negotiation, and game theory; multiagent planning, learning, and control; and multiagent systems engineering, modeling and simulation.