This is a description and personal view of the British Technology Index. The historical development of indexing and classification thought provides a framework for description, and emphasis is laid on the debt to Ranganathan’s ideas of synthesis, facet analysis and chain indexing. Although an analytically derived index, B.T.I. has, as an authority file, many points of similarity with the thesaurus of co-ordinate indexing, and these are illustrated. The use of the computer both for typesetting and as an aid to compilation is considered. Some difficulties that users are likely to experience are explained since these, once appreciated, may be overcome. Some suggestions for further development are made. Finally, a brief analysis of six months of an indexer’s time is given.