This publication presents international consensus principles useful in the design of a research reactor. It is complemented by Safety Series No. 35-S2, Code on the Safety of Nuclear Research Reactors: Operation. Both publications provide basic principles and requirements for the safety of research reactors and critical assemblies, including the essential safety requirements for siting, quality assurance and regulatory control. These codes supersede the 1984 edition of Safety Series No. 35, Safe Operation of Research Reactors and Critical Assemblies.