Carotenoid pigments, including hydrocarbons such as beta-carotene or xanthophylls such as lutein and zeaxanthin, are very widely distributed in nature, where they play an important role in protecting cells and organisms against the harmful effects of light, air, and sensitizer pigments. This process has been demonstrated in bacteria, algae, plants, animals, and even in humans in the light-sensitive disease, erythropoietic protoporphyria. The primary mechanism of action of this phenomenon appears to be the ability of carotenoids to quench excited sensitizer molecules as well as quench 1O2. In addition to this protection, and potentially of even greater biological importance, is the fact that carotenoids can also serve as antioxidants under conditions other than photosensitization. This review presents the data available indicating the extent of this important function. Antioxidant action can be documented in both enzymic and nonenzymic systems, and has been reported in subcellular, cellular, and animal studies. In fact, the many reports indicating that carotenoids may possess some anticarcinogenic properties may well be related to their ability to interact with and quench various radical species that can be generated within cells.