Abstract: Theoretical issues and empirical evidence concerned with the perception and avoidance of impending collision were discussed. A theoretical framework was developed, based on J. J. Gibson's (1950, 1958, 1939) concepts of ecological optics and stimulus information. A series of experiments was performed with invertebrate and vertebrate Ss; several stimulus variables were manipulated, and several hypotheses derived from the theoretical framework were tested. It was found that most animals respond avoidantly and directionally to the abstract visual stimulus property of accelerated magnification of a dark form in the field of view, which specifies the approach of an object and impending collision. Such behavior was found to be relatively independent of shape and magnification rate (with some exceptions) and is apparently not a product of associative learning in some species.