This book constitutes the revised selected papers from the 15th European Conference on Multi-Agent Systems, EUMAS 2017, and the 5th International Conference on Agreement Technologies, AT 2017, held in Evry, France, in December 2017. The 28 full papers, 3 short papers, and 2 invited papers for EUMAS and the 14 full papers and 2 short papers for AT, presented in this volume were carefully reviewed and selected from a total of 76 submissions. The papers cover thematic areas like agent-based modelling; logic and formal methods; argumentation and rational choice; simulation; games; negotiation, planning, and coalitions; algorithms and frameworks; applications; and philosophical and theoretical studies.