A model is proposed to explain the effects of line motion on visual sensitivity. The model incorporates recent evidence for parallel ‘pattern’ and ‘flicker’ systems. An element of either system consists of a spatial filter followed by a temporal filter; the characteristics of these filters were derived from previous psychophysical measurements. The model may be used to demonstrate why visual sensitivity to optimum triangular-wave motion is greater than for optimum square-wave motion, which, in turn, is greater than for on-off presentation. The predictions of the model are in reasonable agreement with threshold-amplitude data for triangular-wave and square-wave motion; the model is also consistent with threshold-frequency data for small-amplitude sinusoidal motion.