The Cambridge crystallographic database comprises files of bibliographic, chemical connectivity and numeric structural data, for organics, organometallics and metal complexes studied by X-ray and neutron diffraction. The files, covering the literature from 1935 and maintained on a current basis, presently contain information on some 25 000 structural studies. Certain categories of information, particularly bibliographic, are disseminated in printed form via the Molecular Structures and Dimensions series. The full potential of the total database depends, however, on its response to specific user queries. The present paper describes the computer-based search and retrieval system, with special reference to question coding. The retrieved data may then be used for extensive and systematic geometric analysis, and for the visual display of crystal and molecular structures. The availability of the database is also discussed.as well as the establishment of abstracting, input and evaluation procedures. The utilization of sections of the database in the production of the reference-book series Molecular Structures and Dimensions (MSD) (Ken-nard &