Illusions of attitude and motion result in spatial disorientation and have been shown to contribute substantially to the frequency of accidents in aviation. Such illusions are normal responses to particular situations, and they involve both ocular and labyrinthine mechanisms, especially the latter. Because they are normal reactions, thorough indoctrination is needed to convince the pilot that labyrinthine and other proprioceptive sensations are totally unreliable, and careful training is needed to induce the proper reliance on flight instruments. Even so, an overcomplicated cockpit layout, extremely high speed, bad weather, and difficult tactical conditions have frequently resulted in rejection of the instrument information by the pilot and have rendered him even more susceptible to disorienting sensations. Eighteen cases are here summarized to illustrate the resulting dangers. Increased effort should be exerted to deal appropriately with all factors in human physiology and in aircraft design that contribute to spatial disorientation as a hazard of flight.