This Safety Guide, part of a set of publications in the IAEA Safety Series dealing with all the important areas of research reactor safety which includes Safety Standards, Safety Guides and Safety Practices, develops the general concepts presented in Safety Series Nos 35-S1 and 35-S2 and should be read in conjunction with them. This Safety Guide presents guidelines for the safe utilization and modification of research reactors to ensure that these projects are implemented without undue risks to personnel, the public, the environment or the reactor. While the Safety Guide is most applicable to existing reactors, it is also recommended for use by organizations planning to put a new reactor into operation. Contents: 1. Introduction; 2. Organization and responsibilities; 3. Safety assessment, categorization and approval routes; 4. General and specific safety requirements for design; 5. Pre-implementation phase of a utilization or modification project; 6. Implementation phase of a utilization or modification project; 7. Post-implementation phase of a utilization or modification project; 8. Operational safety requirements for experiments; 9. Safety considerations in the handling, dismantling, post-irradiation examination and disposal of experimental devices; 10. Safety aspects of out-of-reactor installations; 11. Quality assurance of experiments and modifications; Annex I: Categorization criteria; Annex II: Justification of a project.