Motion sickness occurs in a wide variety of circumstances involving real or apparent motion, many of them novel and man-made. Significantly, voluntary body movements rarely result in symptoms, and the likelihood of motion sickness is greatly reduced by having control of the vehicle in which one is riding. The unifying theory of Reason [1978] proposes that motion sickness results when there is a mismatch between predicted and actual sensory inputs. Hence, the less predictable the sensory input (because it results from an externally imposed motion, or from motion which results in errors of neural transduction), the more likely that motion sickness will develop. With continuing exposure, predictability increases and adaptation to motion sickness occurs.