A model of human motion perception is presented. The model contains two stages of direction selective units. The first stage con(cid:173) tains broadly tuned units, while the second stage contains units that are narrowly tuned. The model accounts for the motion af(cid:173) tereffect through adapting units at the first stage and inhibitory interactions at the second stage. The model explains how two pop(cid:173) ulations of dots moving in slightly different directions are perceived as a single population moving in the direction of the vector sum, and how two populations moving in strongly different directions are perceived as transparent motion. The model also explains why the motion aftereffect in both cases appears as non-transparent motion.