Much effort has been expended in evaluating the relative antioxidant potency of carotenoid pigments in both in vitro and in vivo experiments. It is quite clear that in vitro, carotenoids can inhibit the propagation of radical-initiated lipid peroxidation, and thus fulfill the definition of antioxidants. When it comes to in vivo systems, it has been much more difficult to obtain solid experimental evidence that carotenoids are acting directly as biological antioxidants. In fact, under nonphysiological circumstances, carotenoids may act as prooxidants. These results can be modified by altering the oxidant stress, the cellular or subcellular system, the type of animal, and environmental conditions, such as oxygen tension. Results of this type raise the question as to whether it is still appropriate to group the carotenoids with such antioxidant vitamins as vitamin E and vitamin C. Thus, the biological properties of the carotenoids may be much more related to the products of the interaction of carotenoids with oxidant stress, that is, such breakdown products as apocarotenoids and retinoids.