Carotenoids Carotenogenic Organismsare primarily C40-compounds consisting of eight isopentenylpyrophosphate units, although C30- and C50-carotenoids consisting of six and ten units, respectively, are also found. Some are carotenoid glycosides, carotenoid glycosyl fatty acid esters, or carotenoid fatty acid esters, and more than 850 structurally defined carotenoids are found in nature. They are synthesized by oxygenic phototrophs (land plants, green algae, brown algae, red algae, and cyanobacteria), anoxygenic phototrophic bacteria (purple bacteria, green sulfur bacteria, chloroflexus, and heliobacteria), some eubacteria, some archaea, and some fungi. Most animals also contain carotenoids that are obtained from foods. Some animals can modify them, such as by oxidation, reduction, and elimination. The types of carotenoid present in an organism depend on variation in carotenogenesis pathways, different characteristics of carotenogenesis enzymes, and the presence or absence of certain carotenogenesis genes. Although some enzymes of the major carotenogenesis pathways have been characterized, some major and minor pathways remain unidentified. These variations in carotenoids and pathways can be used for the chemotaxonomic classification of organisms. Carotenoids exert functions such as light-harvesting and protection of high light for photosynthesis and reducing the production of reactive oxygen species for phototrophs and also non-phototrophs, including animals. In this chapter, the distribution of major carotenoids, the major carotenogenesis pathways, the characteristics of carotenogenesis enzymes and genes, and some biological functions of carotenoids are summarized and updated from the first edition.