This Safety Guide is a revision of IAEA Safety Standards Series No. SSG-24, which it supersedes. The current publication provides recommendations on how to meet the applicable safety requirements relating to the utilization and modification of research reactors. The recommendations focus on the categorization, safety assessment and approval of research reactor experiments and modification projects. The publication also incorporates the relevant lessons learned from the accident at the Fukushima Daiichi nuclear power plant and elaborates on interfaces between nuclear safety and nuclear security. Specific safety considerations in different phases of utilization and modification projects are covered, including pre-implementation, implementation, and post implementation phases. Guidance is given on operational safety of experiments, including the handling, dismantling, post-irradiation examination and disposal of experimental devices. The publication is intended to be of use to individuals within the operating organizations of research reactors, regulatory bodies, as well as the experimenters, technical support organizations and other persons involved in utilization and modification projects.