Atomically thin semiconductors, such as monolayer MoSe2 and WSe2, have emerged as novel optoelectronic materials, with coupled spin-valley electronic physics and excitons that are strongly bound at room temperature. It has recently been shown that these materials can form the basis for atomically thin p-n junctions, transistors, light emitting diodes, and low threshold nanolasers. In this presentation, I will discuss optoelectronics and spin effects in heterostructures of MoSe2and WSe2, with type-II band alignment, with the lowest conduction band in the MoSe2 layer, and the highest valence band in the WSe2 layer. Upon optical excitation, electrons transfer to the MoSe2 layer and holes transfer to the WSe2 layer. Due to the strong attractive Coulomb interaction between these spatially separated layers can form interlayer excitons which have many similarities to the spatially indirect excitons of coupled GaAs quantum wells. However, unlike the coupled quantum well system, here the constituent electrons and holes are located in momentum space valleys on the edge of the Brillouin zone. The conduction and valence band valley alignment can be tuned by twist angle between layers to realize optically bright interlayer excitons with an optical selection rule allowing for optical control of the valley degree of freedom. I will discuss the dynamics and spin-valley effects of these bright interlayer excitons.