There are three main sources of colors in natural organisms: pigments, structural colors, and bioluminescence. This third source of color, the fascinating topic of the present chapter, involves light emission from an enzymatic reaction, a phenomenon considered a type of chemiluminescence. The core process of this phenomenon is energy transduction. In contrast to photoluminescence (e.g., fluorescence upon photoexcitation), bioluminescence involves chemiexcitation of a molecule. During the reaction, part of the liberated energy is released in the form of photons rather than heat. Bioluminescence generally involves the enzyme luciferase, which catalyzes the oxidation of a small substrate called luciferin. The product oxyluciferin is generated in its excited state and emits a photon when relaxing to the ground state. The organisms achieve light emission through a purely chemical reaction. Consequently, bioluminescence is particularly suited to nocturnal activity and dark environments such as caves, soils, and the depths of aqueous systems. In contrast, fluorescence requires incident (day) light to generate an excited-state emitter. The Greeks mentioned the so-called cold light of marine species (Aristotle, De Anima, ~350 BC, Book II, Chapter 7; Harvey 1957). The first book devoted to bioluminescence and chemiluminescence was published in 1555 by Conrad Gesner (Carter and Kricka 1982). In the seventeenth century, Robert Boyle showed that oxygen was involved in the process (Longkumer and Kumar 2018). Later in the nineteenth century, Raphael Dubois performed a significant experiment where he extracted the two key components of a bioluminescent reaction and was able to generate light. He proposed the terms luciferine (from the Latin Lucifer meaning “light-bringing”) and the corresponding heat-labile luciferase (Fig. 18.1).