The increased demand for library materials resulting from improved bibliographic access due to automation, and auditors' demands for more accurate accounting of library assets/holdings, have made it increasingly important that the discrepancy between the catalog and the materials in the stacks be reduced. This paper identifies many of the benefits associated with an extensive shelf inventory project in large academic and research libraries. It also describes how to determine many of the associated costs and provides a set of procedures with flowcharts. Topics covered include: types of inventories; conducting a pilot inventory; conducting an inventory (developing inventory procedures, in-stack inventories, out-of-stack inventories, search procedures, and record-keeping and statistics); and concluding remarks. Examples from an inventory conducted at the University of Kansas illustrate the procedures, and results of a survey of Association of Research Libraries (ARL) institutions are cited. A selected bibliography concludes the paper. (THC)