This book constitutes the refereed proceedings of the 20th International Conference on Principles and Practice of Multi-Agent Systems, PRIMA 2017, held in Nice, France, in October/November 2017. The 24 revised full papers presented together with one abstract of a keynote talk and 11 short papers were carefully reviewed and selected from 88 submissions. The intention of the papers is to showcase research in several domains, ranging from foundations of agent theory and engineering aspects of agent systems, to emerging interdisciplinary areas of agent-based research.

This book constitutes the proceedings of the 15th International Workshop on Computational Logic in Multi-Agent Systems, CLIMA XV, held in Prague, Czech Republic, in August 2014. The 12 regular papers were carefully reviewed and selected from 20 submissions. The purpose of the CLIMA workshops is to provide a forum for discussing techniques, based on computational logic, for representing, programming and reasoning about agents and multi-agent systems in a formal way. This edition will feature two special sessions: logics for agreement technologies and logics for games, strategic reasoning, and social choice.