This discussion of the use of computer output microform (COM) as a feasible alternative to the library card catalog includes a brief history of library catalogs and of microform technology since World War II. It is argued that COM catalogs are to be preferred to card catalogs, online catalogs accessed by terminals, and paper printouts. Advantages and disadvantages of film or fiche formats for a library catalog are explored in relation to retrieval performance, cost, and ease of use, and such features as updating procedures, readability, and possible patron reluctance to use COM are discussed in greater depth. It is concluded that COM appears to be the most viable answer to new demands placed on the library catalog. (SW)