This book constitutes the revised selected papers from the 6th International Conference on Agreement Technologies, AT 2018, held in Bergen, Norway, in December 2018. The 11 full papers and 6 short papers presented in this volume were carefully reviewed and selected from a total of 28 submissions. The papers discuss new ideas and techniques for the design, implementation and verification of next generation open distributed systems centered on the notion of agreement among computational agents. They are organized in the following topical sections: AT foundations and modelling of reasoning agents; argumentation and negotiation; coordination in open distributed systems with applications.