This review of the research on subject access to library collections focuses on the problems of and prospects for improved online subject access to library collections. Summaries of the general findings of studies on library catalog use and catalog users and some reasons for the frequent failure of subject searches in library catalogs are followed by a discussion of the use of "failure analysis" as a technique in studies of automated information retrieval systems. The advantages and disadvantages of free text searching are reviewed, the feasibility of using the Preserved Context Indexing System (PRECIS) to supplement Library of Congress Subject Headings is briefly considered, and some of the conclusions drawn from studies of library users' needs are presented. Research on the enrichment of cataloging records using free text descriptors, on enhancing currently used subject access systems such as Library of Congress Subject Headings, and on ensuring the effectiveness of the user interface with an online catalog is also discussed. Six recommendations are made for the improvement of subject access in online catalogs, and a 41-item reference list is included. (JL)