The ability to convert several low-energy photons into a single higher-energy photon is of significant importance in diverse fields ranging from imaging and biological labeling to optoelectronics and photovoltaics. The possibility to realize this phenomenon on the nanoscale can provide an additional degree of freedom in engineering electronic properties of materials and would allow deliberate manipulation and optimization of the upconversion processes. The purpose of this chapter is to provide a review of physical mechanisms that govern the photon upconversion in semiconductor nanostructures. Taking into account a large number of comprehensive reviews on this topic, our main focus is on photon upconversion mediated by defects, which is far less explored so far but provides a viable and attractive alternative for achieving efficient photon upconversion without involving doping.