Two aspects to be considered in designing a storage system for library materials are: the fraction of the collection which is to be stored; and the criteria to be used for selecting materials for storage. This study demonstrates that for a given selection criterion least-cost storage quantities can be assessed by balancing the large circulation costs and the small shelving costs associated with stored materials. Two storage criteria, one based upon the age of the materials and the other utilizing the individual book usage rates, are discussed and compared. For the age policy the objective is the determination of a least-cost critical age at which materials are transferred to storage and which defines the fraction of the collection that should be stored. The model assumes that the circulation rate of the books declines with age according to an average geometric pattern of obsolescence. (Author)