The actual and potential contributions of human factors to the smooth and efficient functioning of systems for the ground control of aircraft are described and discussed. These systems are concerned with air traffic control or air defense, which fulfill different roles but share many human factors problems. The nature of the human factors contribution at each stage in the evolution of a ground control system is described. Much work has dealt with displays, controls, or communications, or with man's role in complex man-machine systems. Individual differences are considered in relation to selection, training, and screening procedures, and some of the differences which appear most relevant in systems are discussed. Both traditional and new methods for measuring operators are examined in terms of their merits and limitations. The factors of potential relevance to task performance are very numerous, and certain advances in other applied or academic contexts may be adapted with profit to ground control systems. Many of the proposed solutions to human factors problems have been based on limited evidence, mainly because man is treated primarily as a system component. Numerous studies have produced findings related to a specific task, but few widely applicable facts have emerged. Some probable future trends in the application of human factors to ground control systems are suggested. (Author).