Evaluation of library service effectiveness is defined in terms of providing objective information to library policy makers and program managers on the effectiveness and cost of alternatives, 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 own libraries in order to improve the availability of information for library decision making. A selective bibliography on evaluation is appended. (Author/JPF)