Semiconductor nanowire is a very useful platform for nanoscience and engineering research. The capability of rational synthesis and band engineering allow control over the electrical, optical, and mechanical properties at nanoscale, and thus enable enormous opportunities in electronics, optoelectronics, renewable energy, biosensing, and nanomedicine. In this talk, I will first present the ultrahigh sensitive nanowire photodetectors and the use of nanoengineered artificial photoreceptors for prosthetic device applications with sensitivity beyond human eyes. Then I will discuss results of nanowire photovoltaic devices (solar cells and photoelectrochemical cells) with significant potential for enhanced device performance and energy conversion efficiency at reduced fabrication cost.