An air traffic control system, for use by a controller controlling a plurality of aircraft held vertically separated in a stack, the system comprising at least one processor; a display device for the control generating a display controlled by said at least one processor, and at least one device for selectively receiving, from said aircraft, an indication of their intended future altitudes; in which said processor is arranged to receive such intended altitude data; to compare said intended altitude data with current altitude and/or intended altitude data of other aircraft; and to generate said display on said display device so as to list said plurality of aircraft, to highlight a first part of the display relating to a first aircraft whose intended altitude overlaps with the current or intended altitude of at least one said second aircraft, and to highlight also a second part of the display relating to said second aircraft.