The properties of movement detectorsin man, inferred from psychophysical tests, are remarkably similar to those found in other animals such as the frog, pigeon, rat, rabbit, cat, and monkey. The smallest fields in man are about 2° in diam., increasing to about 20° in the periphery. The fields are directionally specific, and exhibit surround inhibition. Preferred directions for centers are horizontal and vertical; the inhibitory surround is not directionally selective. Optimal sensitivity to speed for foveal objects occurs at about 3 to 6°/sec, and the contralateral pathway is about 1.4 X more sensitive to motion than the ipsilateral route. The inferred properties of motion detectors appear to represent the behavior of a subcortical system, presumably involving the superior colliculus or pretectum. Under certain conditions, this movement detection system may be shown to interact with stereoscopic mechanisms that are believed to be cortical.