The emission of light by organisms or minerals has fascinated observers since ancient times. These emissions have multiple and varied origins. Gathered under the generic term of luminescence, they are generally distinguished by the nature of the excitation. Etymologically, “luminescence” derives from the Latin word “lumen” (English: light) and was first introduced in 1888 by the physicist Eilhardt Wiedemann as “Lumineszenz” (Lakowicz 2006; Valeur and Berberan-Santos 2012). Today, “luminescence” describes all phenomena of light emission which are not caused by temperature increase (“incandescence”). In principle, a physical system is brought into an excited state by an external energy transfer and emits light when relaxing to its ground state. Most types of luminescence are listed in this chapter (see Table 21.1) according to the nature of their different excitation modes. In the case of bioluminescence (see Chap. 18 ), light is produced directly by the enzymatic reactions of a living organism, without external excitation.