This book constitutes the refereed proceedings of the 18th International Conference on Group Decision and Negotiation, GDN 2018, held in Nanjing, China, in June 2018. The field of Group Decision and Negotiation focuses on decision processes with at least two participants and a common goal but conflicting individual goals. Research areas of Group Decision and Negotiation include electronic negotiations, experiments, the role of emotions in group decision and negotiations, preference elicitation and decision support for group decisions and negotiations, and conflict resolution principles. The 15 full papers presented in this volume were carefully reviewed and selected from 143 submissions. They were organized in topical sections named: theoretical concepts of group decision and negotiation; decision support and behavior in group decision and negotiation; and applications of group decision and negotiations.