Libraries assemble very large quantities of materials. These collections perform three quite different roles: archival, dispensing, and bibliographic. The bibliographic role of the collection is compared with bibliographies and catalogues. The distinction between materials and collection development is basic. Collection development in libraries is analogous to file organisation in computing systems and, although commonly viewed narrowly as selection for acquisition, includes a range of decisions which determine the profile of any collection. The rise of remotely‐accessible materials makes possession less important relative to access, has important consequences for all three roles of collections, and indicates a shift in emphasis away from collection development and conventional catalogues and toward bibliography and cooperation.