[en] This book is in two parts. The first is a Code of Practice for the Safe Operation of Critical Assemblies and Research Reactors, prepared as a result of a meeting of experts which took place in Vienna on 20-24 May 1968. The Code has been prepared by the International Atomic Energy Agency in co-operation with the World Health Organization, and its publication is sponsored by both organizations. In addition, the Code was approved by the Board of Governors of the International Atomic Energy Agency on 16 December 1968 as part of the Agency's safety standards, which are applied to operations undertaken by Member States with the assistance of the Agency. The Board, in approving the publication of the present book, also recommended Member States to take the Code into account in the formulation of national regulations and recommendations. The second part of the book is a Technical Appendix to give information and illustrative samples that would be helpful in implementing the Code of Practice. This second part, although published under the same cover, is not part of the Code. An extensive Bibliography, amplifying the Technical Appendix, is included at the end.