During the past twenty-five years the aviation scene has progressed from one in which relatively small numbers of aircraft flew from point of origin to destination with the guidance afforded by very limited M/F range facilities and a great deal of individual navigational skill on the part of the operating crews. Air traffic control functioned only as a means of sorting out the order of take-off and landing. Moving forward to the present we see the already high density traffic serving the population centres of the world continuing to increase at a rate of around 10 per cent per annum. Both station referenced and self-contained navigational systems proliferate and control of traffic must be almost total if collision accidents are to be avoided. During the same time period aircraft speeds have increased threefold and necessitated a consequent improvement in the ability of the air traffic control service to react quickly and precisely to any conflict situations, either by detection in the strategic/planning case or resolution in the tactical frame by direct intervention before critical situations can develop.