Since 1982 the Naval Training Systems Center, with support from the Office of Naval Technology and the Naval Air Systems Command, has been investigating the occurrence of simulator sickness in Navy flight simulators. Simulator sickness is defined as that group of symptoms experienced by crew members in connection with maneuvers in flight simulators which would not be experienced by those same crew members in aircraft. Symptoms include nausea, dizziness, general discomfort, eyestrain, headache, disequilibrium, and pallor. In rare cases there have been aftereffects (e.g., visual flashbacks and disorientation) that have occurred up to 12 hours after exposure to the simulators. This paper reviews results of the Navy's research and subsequent recommendations that have been provided to Navy trainer acquisition managers who formulate specifications for future flight simulators. The rationale for these suggestions is derived from literature reviews, field observations, laboratory experimentation, and the results of a biomedical panel convened to address the simulator sickness problem.