Peer reviews are a standard co-operative OECD working tool that offer member countries a framework to compare experiences and examine best practices in a host of areas. The OECD Nuclear Energy Agency (NEA) has developed a proven methodology for conducting peer reviews in radioactive waste management and nuclear R&D. Using this methodology, the NEA Radioactive Waste Management Committee’s Working Party on Decommissioning and Dismantling (WPDD) developed the present guide as a framework for decommissioning cost reviewers and reviewees to prepare for and conduct international peer reviews of decommissioning cost estimate studies for nuclear facilities. It includes checklists that will help national programmes or relevant organisations to assess and improve decommissioning cost estimate practices in the future. This guide will act as the NEA reference for conducting such international peer reviews.