A review of computer systems simulation is presented. Existing and proposed programs for simulating computer systems are described and recommendations are made about the desirable characteristics of a simulator. Attention is primarily focused upon those simulators that may be used to study time-sharing, multiprogramming, and multiprocessing systems. It appears that a higher order, special purpose simulation language is needed for simulating computer systems. CSS and LOMUSS II seem to be representative of the most promising type of computer system simulators.