The past few decades have been an era of rapid growth and expansion for university and research libraries, yet their standards of library services have declined. Specifically, more people more often cannot get the information they seek from the library. The main factors in this decline in service are the rapidly increasing numbers both of volumes and of patrons and the changing nature of demand. Libraries still conduct their operations as if their strategy were to store and provide access to books, periodicals, papers and other materials. Their current strategy should be to supply information. This change in goals has implications for the library's acquisition, shelving and retention policies. The library's resources may be thought of as Information Message Units (IMU). The value of each IMU can be defined only in terms of its worth to its potential users, the members of the university community. A Bayesian approach can be used to calculate the information value of the IMU, thus affecting decisions on both the acquisition of volumes and the maintenance of collections. Although developed in the context of an individual library system, this approach can also be applied to networks of library systems. (Author/SL)