INIS - the International Nuclear Information System - is planned to serve the information needs of the world's nuclear scientists and technologists. Records of newly published literature in the nuclear field are prepared for input in national and regional centres. These records are sent to the International Atomic energy Agency in Vienna, where they are processed by an electronic computer and merged to create a master file. Computer-based information systems operate successfully only if the input is prepared with a high degree of consistency and accuracy. Because the preparation of input for INIS is highly decentralized, some people have asked whether, in practice, the necessary standards can be maintained. The object of the present document is to explain the tools for indexing which are at the disposal of the specialist indexer, to establish the rules to be used in indexing, to give general suggestions and aid for the selection of appropriate descriptors as well as to lay down guidelines and suggestions for choosing subject headings under which to list entries in the printed issues of Atomindex.