The phenomenon of photoluminescence – light emission promoted by absorption of light – is displayed by a wide range of dyes and other molecules, as well as various materials. It is a very important phenomenon that is exploited in a broad range of technologies. For example, in many very important analytical methodologies especially in biological and medical applications, for detection, diagnosis, and treatment of diseases and for following biological processes in general; in more general assays for a wide variety of chemical entities including explosives; for the detection of forgeries, or to establish authenticity; and in some forms of lighting. The chemical nature of photoluminescence and its applications are detailed in this chapter.