Most electronic devices exploit the electric charge of electrons, but it is also possible to build devices that rely on other properties of electrons. Spintronic devices, for example, make use of the spin of electrons(1,2). Valleytronics is a more recent development that relies on the fact that the conduction bands of some materials have two or more minima at equal energies but at different positions in momentum space(3-5). To make a valleytronic device it is necessary to control the number of electrons in these valleys, thereby producing a valley polarization(6-11). Single-layer MoS2 is a promising material for valleytronics because both the conduction and valence band edges have two energy-degenerate valleys at the corners of the first Brillouin zone(12). Here, we demonstrate that optical pumping with circularly polarized light can achieve a valley polarization of 30% in pristine monolayer MoS2. Our results, and similar results by Mak et al.(13), demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS2 monolayers.