III-V compound semiconductor nanowires (NWs) are being developed for the next generation of optoelectronic devices such as photodetectors, photovoltaics, betavoltaics and thermoelectrics. The self-assisted vapor-liquid-solid method is now a well-established technique for the growth of III-V NWs on silicon substrates. In this method, an array of holes in a SiO2 film is used for metal droplet formation, which seeds the growth of vertically oriented NWs within a periodic array. The free lateral surfaces of NWs allow elastic relaxation of lattice misfit strain without the generation of dislocations, permitting unique heterostructures and the direct integration of III-V materials on inexpensive silicon substrates. Furthermore, NWs permit high optical absorption due to an optical antenna effect. The optical absorption in NW arrays can exceed that due to a thin film of equivalent thickness, enabling high efficiency NW-based photovoltaic devices. Optical resonances that depend on the NW diameter allow multispectral absorption for infrared camera applications. Some of the challenges associated with NW materials and devices will be illustrated.