As our society moves increasingly from an era of affluence to one of scarcity, the evaluation and assessment of publicly supported services will become even more crucial and necessary to ensure that public resources are being expended in the most effective manner. Evaluation of library service effectiveness is defined in this paper in terms of providing objective information to library policy makers and program managers on the effectiveness and cost of alternative as well as existing programs and services. The elements of policy and program evaluation are discussed, and the need to view evaluation as a continuous process is emphasized. Library administrators are urged to make a strong commitment to support evaluation efforts within their libraries in order to improve the availability of information for library decision making. A selected bibliography is attached. (Author/KP)