This book constitutes the proceedings of the 12th International Conference on Decision Support Systems Technologies, ICDSST 2026, held in Plymouth, UK, during May 27-29, 2026. The EWG-DSS series of International Conferences on Decision Support System Technology (ICDSST) aims to continue 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. This year, the main focus of the conference was on Decision Support System Technology for Sustainable and Resilient Transitions. The 27 full papers included in this book were carefully reviewed and selected from 70 submissions. They were organized in topical sections as follows: Decision Support Systems and Models for Complex Transitions; Decision Support Systems and Models for Energy and Green Transition; Decision Support Systems and Multicriteria Decision Making; Large Language Models and Data-Driven Decision Support; and Decision Tools, Applications and Business Cases.