Developed at the beginning of the decade to serve as a union catalog for identifying the holdings in all University of California (UC) libraries, the MELVYL online catalog as of October 1989 contained 10 million holdings for over 5 million monographic titles of print and nonprint materials, and more than 1.2 million periodicals holdings which represent nearly 650,000 additional titles. MELVYL, which is constantly evolving to meet user needs, currently provides a union catalog of UC's 100 libraries, the California State Library, and the libraries of the 32 California State Universities, Stanford University, The University of Southern California, the Center for Research Libraries, and the J. Paul Getty Center for the History of Art and the Humanities. Several features of the system are designed to instruct users in accessing the catalog, including "News,""Help," and "Explanation" screens. Supplemental information and instructional materials are provided by two serial publications that are also available on line. The catalog is operational in three modes--"Lookup," the simple mode; "Command," which is faster and more flexible; and "Experimental," which can be added to either of the other two. MELVYL has the following advantages: it is fast, allowing combined concepts and cumulative searching; it is both current and retrospective; and it can be limited by dates, languages, locations, and boolean logic. A major disadvantage is a high rate of failure in subject searching, which may be attributed to the fact that only Library of Congress subject headings are used, and access to the subject heading list is not yet available directly on line. (SD)