Abstract: An air traffic control system, for use by a controller controlling multiple aircraft, comprising a processor, an input device and a display device, further comprising: trajectory prediction means for calculating a trajectory for each aircraft, for inputting aircraft detected position data, and for recalculating the trajectories based on said position data, and conflict detection means for detecting, based on the trajectories, future circumstances under which pairs of aircraft violate predetermined proximity tests, and for causing a display on the display device indicating said circumstances, further comprising means for inputting instruction data corresponding to instructions issued by the controller to an aircraft, and in which the proximity indication means is arranged to use a first proximity test and a second, more restrictive, proximity test; and in which the system is arranged to display a symbol representing pairs of aircraft which violate the second test in a first display mode, and those which violat