The air traffic controller provides a service and can do so effectively only with the cooperation of his customers, namely the pilots of many different types of aircraft. For the purpose of this paper it is assumed that pilots do not wittingly take avoidable risks and are fully capable of finding their way from one place to another by the shortest available route. According to I.C.A.O., A.T.C. exists to provide ‘safety and expedition’, but, left to his own devices, the pilot can take care of his own ‘expedition’ so that the main function of A.T.C. is to provide a safety service. When A.T.C. has to regulate flights for safety reasons, however, it may have to ask a pilot to deviate from his chosen flight path. It then has to assume a measure of responsibility for expedition, but safety is still the prime consideration.