The basic process of air traffic control is a complex control feedback system, with human servomechanisms at its input (traffic controller) and output (airplane pilot) ends. Controller and pilot interact via communication links, receiving and transmitting information displayed almost exclusively to the visual and auditory senses. Human abilities and limitations for handling the required data are discussed with respect to present and proposed display techniques. The effects of quantity, rate, reliability (ambiguity) and storage of data are related to air traffic control system effectiveness. The means by which this may be determined are considered from a human factors viewpoint with regard to safety, convenience, and economy.