This Safety Guide provides recommendations for the commissioning of research reactors on the basis of international best practices. The guidance and recommendations of this Safety Guide are applicable to most types of research reactors and fulfil the general requirements on research reactor safety presented in IAEA Safety Standards Series No. NS-R-4, Safety of Research Reactors, as well as those in Safety Series No. 35-G1, Safety Assessment of Research Reactors and Preparation of the Safety Analysis Report. Commissioning is one of the major steps in the life cycle of a research reactor, and appropriate guidance for conducting the process is essential. The emphasis in this Safety Guide is on the commissioning of a new research reactor, but guidance is also provided on the commissioning of new experiments and of reactor modifications.