To avoid any misunderstandings, it should be made clear that in the author's opinion the techniques of Air Traffic Control are not likely to be significantly affected by the future advent of an air-to-air collision avoidance system. This is admittedly a contentious view and this paper is a brief survey of some of the factors which lead the author to hold this opinion. After clarifying what is meant by an air-to-air collision avoidance system, the paper deals first with some of the operational problems, then with some of the technical problems and concludes with general comments.