Apart from visual avoidance of other aircraft by pilots the only system of preventing air-to-air collisions at present is by means of air traffic control from the ground. With the increased speed of modern aircraft and increased traffic density on long-distance routes there is a tendency to demand further extension and development of air traffic control to solve the collision problem. The object of this paper is to examine the problems and limitations of air traffic control as a method of avoiding collisions between aircraft in cruising flight and how these are likely to be affected by future aircraft developments.