Although precise data are unavailable, it is clear that the theft of library materials is a significant problem. However, most thefts of these materials can be prevented. Among the basic security policies that should be practiced are marking valuable materials so that proof of ownership can be established if they are stolen, maintaining accurate cataloguing and inventory records so that an accurate report of missing documents can be filed, and keeping copies of records that can be made available as a means of proving ownership. Accurate recordkeeping is especially important for archival repositories, including the maintenance of accurate inventory records, particularly the calendars of papers for the sets of papers in the collection, retention of the paging call slips with the name of each patron who requests materials, and retention of registration forms filled out by researchers. Some materials should be preserved on microfilm. Libraries and archival institutions should also promote good relations with book and document dealers. To address internal and external theft, institutions must establish procedures applicable to both staff members and visitors. Another action is to lobby for stronger laws for the conviction and prosecution of library crimes. The installation of electronic and mechanical security systems is a widely used security measure. 82 references