This book serves as the Proceedings of the ICEER 2025 conference. It contains the full papers of the 12th edition of the International Conference on Energy and Environment Research, ICEER 2025, which took place in two events: the first took place in Budapest, Hungary, during July 22–25, 2025, and the second took place in September 12–14, 2025, in Chengdu, China. This book addresses pertinent issues such as decarbonization, renewable energy sources, LCA methodology applications, sustainable approaches for energy products and services, and energy policies. ICEER 2025 focused on the theme “Towards Sustainable Geographies with Net Zero Carbon Economy,” and it was a joint organization of the School of Engineering (ISEP) of the Polytechnic of Porto (P.Porto) and the SCIEI. This book includes all the well-prepared full papers presented at ICEER 2025 that were reviewed by at least two independent reviewers.