Semiconductor nanowires have attracted considerable recent interest due to their unique properties, including their highly anisotropic geometry, large surface-to-volume ratio, and carrier and photon confinement. Currently, tremendous efforts are devoted to the rational synthesis of advanced nanowire heterostructures. Yet, if functional devices are to be made from these materials, precise control over their composition, structure, morphology, and dopant concentration must be achieved. Their fundamental properties must also be carefully investigated since the presence of a large surface and interfacial area in nanowires can profoundly alter their performance. In this article, the progress, promise, and challenges in the area of nanowire heterostructured materials are reviewed, with particular emphasis on the effect of different types of heterointerfaces on device properties.