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. (Please note: this publication will be superseded by SSG-24)