The yearly average number of airliners reported, worldwide, as having been involved in a mid-air collision is less than two. Air traffic control (ATC) aims to hold this rate, despite future traffic growth. Improvements in air-ground communications, e.g. by the use of selectively addressed secondary surveillance radar (SSR Mode S), make possible surveillance of the air situation by ATC computers, thus trapping potentially dangerous discrepancies between ATC plans and reality. The necessary level of confidence makes it unlikely that computers will soon take over from human pilots or controllers. However, as computers are prone to different errors than those of humans there is a good case for mechanised redundancy to assist ATC decision taking.