This book constitutes the proceedings of the 9th International Conference on Decision Support Systems Technologies, ICDSST 2023, held during May 30 – June 1, 2023.The EWG-DSS series of International Conference on Decision Support System Technology (ICDSST) is planned to consolidate the tradition of annual events organized by the EWG-DSS in offering a platform for European and international DSS communities, comprising the academic and industrial sectors, to present state-of-the-art DSS research and developments, to discuss current challenges that surround decision-making processes, to exchange ideas about realistic and innovative solutions, and to co-develop potential business opportunities. The main theme of this year was “Decision Support Systems in An Uncertain World: The Contribution of Digital Twins”. The 21 papers presented in this volume were carefully reviewed and selected from 65 submissions. They were organized in topical sections as follows: DSS models, methods, and tools; DSS for business performance and stakeholders; DSS applications for sustainability in health, energy and transportation; and DSS users and successful adoption.

This book constitutes the refereed proceedings of the 23rd International Conference on Group Decision and Negotiation, GDN 2023, which took place in Tokyo, Japan during June 11–15, 2023.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. This year’s conference focusses on multimodal interactions. The 11 full papers presented in this volume were carefully reviewed and selected from 102 submissions. They were organized in the following topical sections: Taking a step back: Critically re-examining technology interactions with group decision and negotiation; preference modeling and multi-criteria decision-making; and conflict modeling and distributive mechanisms.