When the problem of collision in the air is discussed, it is usual to start by pointing out the enormous closing speed of two modern aircraft meeting head-on, and to conclude from this that avoidance on the ‘see and be seen’ principle has ceased to be possible. The fact is, however, that the great majority of mid-air collisions (about 85 per cent) occur within five miles of an airport and the typical case is not the head-on one, but the case in which the two aircraft crab into one another from a direction which may be anywhere around the whole enclosing sphere. Since the field of view of the aircrew covers only about 20 per cent of the enclosing sphere, the aircrew of colliding aircraft seldom see each other. It would seem, therefore, that the ‘see and be seen’ principle never did afford much protection, even when speeds were low. In other words, the fact that the number of mid-air collisions in Europe has hitherto been small is not primarily due to seeing and evading, although this sometimes happens, but to the fact that the airspace is very large compared to the volume of all the aircraft in it at any given time. However, as traffic densities go up, the risk rapidly increases, and in congested airspace, such as that around New York, the problem of avoiding collision has already become acute. In the period 1948–57, there were 159 mid-air collisions in the United States, and many of these made headlines in the world press. One can imagine the public outcry if two large transports were to collide over a housing estate; but unless something effective is done, something like this will presumably happen eventually. At very high altitudes the ‘see and be seen’ principle certainly fails, by day, because the speed will be high, and in addition, the range at which a pilot can see an object the size of an aircraft may be less than 1½ miles due to what is sometimes called ‘high-altitude myopia’.