Decommissioning planning reshapes project outcomes by integrating safe, efficient strategies from inception through operation. Decommissioning is a complex stage in research reactor projects, the safe and efficient implementation of which can be greatly facilitated if it is considered from the early stages and throughout the project's lifespan. This publication provides guidance on facilitating decommissioning during the design, construction and operation stages of a research reactor's lifetime, and on managing objectives and requirements during the transition period. It presents good practices and lessons learned in the planning and preparation for decommissioning, as well as information relating to regulatory and management aspects. The publication is intended for individuals and organizations responsible for the design and operation of research reactors, as well as regulatory bodies, technical support organizations and decommissioning planners.