Various models that have been devised to evaluate the performance of information retrieval systems are reviewed. Such models fall into three categories: (1) evaluation from the viewpoint of the user, who is primarily concerned with how well the system responds to his query; (2) evaluation from the system operator's standpoint, which focuses on the operation of the system; and (3) a merging of these two points of view. Parameters considered include system costs, recall, precision, form of output, and response time. Although most of the models described are mathematically derived, no attempt is made to elaborate upon the equations. A bibliography is attached. (JD)