It is not an easy matter to consider the effects of stress upon people, especially those working in a professional environment. The work which formed a basis for this chapter was connected with RAF pilots in the Night/All-Weather force It was their job to take off under poor instrument weather conditions, day or night, find a target on radar, identify it visually in the dark, and then get back again to base The author was an instrument rating examiner on a night/all-weather squadron at the time He was puzzled by the difference in performance of pilots - who were highly skilled and accurate in their work - when it came to their annual instrument rating tests Some of the pilots failed the test, some were au sick, some showed symptoms of "testitis" Those symptoms disappeared when the author threw away the standard instrument flying tests and replaced them with others which simulated operational tasks. Later, after publication of his investigations and the results of some of the experiments, the methods were adopted by the Institute of Aviation Medicine at Farnborough, during the course of a disorientation programme with several pilots who had disorientated during normal flying duties. In every case, as far as the author can remember, the unwitting victim of disorientation was sensitive, imaginative, and often artistic. This accorded with something noticed on the squadrons that the pilots who did not suffer particularly from "testitis" were the men whose interests were more extroverted, who did not inhabit to any significant extent a world of the imagination and intellect Later still, when involved in service diving training and operation, the author found marked similarities between the effects of difficult diving and difficult flying. In both cases the individual was up against a hostile environment. His only protection was equipment, which had to be used properly for safety However, stress could cause him to make mistakes, some of which could be fatal Fatigue, for example, is a consequence of stress - as well as of other factors A classic example of the effect of fatigue was that of a young night-fighter pilot taking off on his third one-and-a-half hour sortie on the same night He had to turn left while climbing and change his radio to the radar controller's frequency. The radio channel selector was beside his right buttock, which meant turning his head to the right and looking down. Coriolis component of acceleration then entered the picture, affecting the liquid in the semi-circular canals in the ears Motion of the liquid bent the fine ham in the canals, giving spurious signals about his equilibrium, which tiredness prevented him from sorting out and overcoming. He barrel-rolled into the ground Ks young widow - maybe through stress, as well as being an orphan - later committed suicide. The author introduced Coriolis effects into the instrument rating test as a result of this accident.