Since World War II, the use of simulators for training has increased, due primarily to cost, maintenance, availability and safety. Orlansky and String (1977 Orlansky and String (1979) have summarized simulator training effectiveness and cost effectiveness. Technologically advanced simulators, such as those for training air combat maneuvering, air cushion vehicles, and Skylab crews, are now commonplace. Unfortunately, there has been a recent increase in reports of discomfort and distress associated with the use of flight simulators. The data from a Navy sponsored survey of simulator sickness are described and a theoretical model is proposed.