Motion sickness is commonly elicited in situations involving unusual patterns of acceleration (e.g., sea sickness, space motion sickness) or sensory stimulation (e.g., optokinetic drums). Symptoms are also evoked in flight and car simulators where the experienced motion and the inertial motion of the body do not correspond. Motion sickness is now being reported in virtual environments, especially those involving dynamic patterns of apparent self-motion. Such environments embody sensory and inertial discordances that are for physical situations of stimulation most provocative. Motion sickness embodies a wide constellation of signs and symptoms, not just nausea and vomiting. In fact, drowsiness, eye strain, lack of initiative, and fatigue are symptoms of motion sickness commonly elicited in relatively unprovocative environments. Progress in understanding and predicting the occurrence of motion sickness has been hampered by the lack of an adequate theory. This talk will include discussion of situations that elicit motion sickness, how symptoms vary for different situations, how adaptation can be hastened, and how it is possible to have multiple adapted states for different environments.