Atomically thin transition metal dichalcogenides (TMDs) have emerged as a new class of 2D crystals beyond graphene. In this talk, I will discuss properties of MoS2 as a prototype for semiconducting TMDs. Recent studies show that this material crosses over from an indirect gap to a direct gap semiconductor at the monolayer thickness. As is common for lower-dimensional materials, many-body interactions are found significant in MoS2 monolayers, as we demonstrate the observation of trions as an example. Monolayer MoS2 also possesses an interesting electronic structure, which permits the production of valley polarization by optical pumping with circularly polarized light, opening new possibilities for valley and spin-based physics and applications.