Carotenoids can be attacked enzymatically at almost every position in the molecule. On the other hand, under dark nonoxidative conditions, they can be very stable. Thus, the precise chemical and biological environment is of crucial importance in determining whether, and how, they are transformed. Three important biologically active derivatives of carotenoids, vitamin A, trisporic acid and abscisic acid, all of which serve as hormones in appropriate cells, are, or can be, formed from precursor carotenoids. Highly active forms of all three hormones are 9-cis isomers. In all cases, the products are involved both in light-induced reactions as well as in cellular differentiation, often related to sexual maturation. Each of these hormones is formed by an initial dioxygenase attack on the central conjugated chain of carotenoids followed by a series of specific reactions. Indeed, when various carotenoids of different structure are studied in humans, each shows a characteristic, if not unique, set of metabolites. Thus, generalizations about carotenoid metabolism must be constrained by precise metabolic information about given compounds. In dealing with the manifold biological activities of carotenoids, it is useful to categorize them as functions, actions or associations. In the hope of gaining greater insight into the relationship between carotenoids, health and longevity, these distinctions should be helpful.