Porous silicon nanostructures have attracted a great deal of interest during the past few years, due to their many remarkable properties. The high-efficiency visible photo- and electro-luminescence of this material opened the way to the development of silicon-based optoelectronic devices fully compatible with standard industry processes. In addition to these luminescent properties, nanostructured porous silicon shows a variety of other interesting properties, including tunable refractive index, low light absorption in the visible, high internal surface, variable surface chemistry, or high chemical reactivity. All these properties, along with its ease of fabrication and the possibility of producing precisely controlled layered structures make this material adequate for its use in a wide range of fields, such as optics, micro- and optoelectronics, chemical sensing or biomedical applications, for example. This article reviews the applications of nanostructured porous silicon that exploit its unique optical properties, as in the case of light emitting devices, filtered photodetectors, optical sensors, and others.