It might be thought that motion perception is simply the outcome of retinal image motion, but in fact this is so only for a stationary observer with unmoving eyes. Retinal image motion can be a consequence of the displacement of an object relative to a stationary environment, of an observer relative to a stationary object or of some combination of both. All these conditions result in changes in the pattern of stimulation at the eyes, but despite these we retain an appropriate representation both of our position in space and that of the object. Occasionally, errors do occur in our perception of motion in the natural environment, and these can be very instructive in understanding the nature of motion perception. It is these errors that have received most attention both historically and in contemporary research, which is perhaps surprising as this focus has diverted attention from the extraordinary ability of perceptual systems to recover motion accurately under widely varying conditions. We, as well as our metaphorical guide dog, can perceive the constant properties of objects (such as their location, motion, size and colour) despite large variations in the pattern of sensory stimulation.