This chapter provides a theory for a clearer understanding of space perception and describes the experiments and tests involved in the theory. A systematic analysis of aerial space and distance should, if correct, have many practical applications to the problems of pilot selection and training. The maneuver of landing a plane is considered an outstanding example in which accurate perceptual judgment is needed by the pilot. If the perception of aerial space and distance are of such importance for the selection and training of fliers, it is obvious that psychological tests and training methods need to be devised with which to select and train them. The cue of aerial perspective as ordinarily described provides a kind of retinal gradient which is a continuous correlate of distance. When a retinal gradient of velocity exists in the way described, perception which results is not that of a visual environment which moves but of a stationary world in which the observer himself moves.