Whenever the subject of air traffic control is discussed reference is made to what are described as ‘areas of high traffic density’. This is a misleading expression because the areas referred to are those in which the traffic density is high in relation to the capacity of the air traffic control system, not to the airspace itself. It is probably true to say that there are in fact only three areas where traffic density is high in relation to the volume of airspace. These are the arrival and departure paths at busy aerodromes and the area occupied by a number of aircraft flying in close formation. Elsewhere the traffic density is not such as to create congestion in the air. It is the traffic control system which becomes overloaded, not the airspace. In this paper an attempt is made to isolate some of the factors giving rise to this state of affairs and to discuss ways of achieving a better state of balance between airspace capacity on the one hand and control capacity on the other.