Of approximately 600 naturally occurring characterized carotenoids, fewer than 10% serve as precursors of vitamin A in mammals. Apart from vitamin A formation, carotenoids show unique physicochemical properties and interesting biological effects. The inverse association between the ingestion of carotenoid-containing fruits and vegetables and the risk of certain forms of cancer has attracted significant attention. In considering the effects of carotenoids in mammals, a distinction should clearly be made among functions, actions and associations. Furthermore, carotenoids might well be classified in relation to defined biological actions, and not just in relation to nutritional activity.