Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more minima in the conduction band (or maxima in the valence band) in momentum space, and if it is possible to confine charge carriers in one of these valleys, then it should be possible to make a valleytronic device(1-4). Valley polarization, as the selective population of one valley is designated, has been demonstrated using strain(5,6) and magnetic fields(7-10), but neither of these approaches allows dynamic control. Here, we demonstrate that optical pumping with circularly polarized light can achieve complete dynamic valley polarization in monolayer MoS2 (refs 11,12), a two-dimensional non-centrosymmetric crystal with direct energy gaps at two valleys(13-16). Moreover, this polarization is retained for longer than 1 ns. Our results, and similar results by Zeng et al.(17), demonstrate the viability of optical valley control and suggest the possibility of valley-based electronic and optoelectronic applications in MoS2 monolayers.