After the discovery of vitamin A in 1913, the yellow pigments of fruits and vegetables were soon implicated as compounds with similar nutritional effects. beta-Carotene was shown to be converted into vitamin A by Moore in 1929, and the chemical structures of both vitamin A and beta-carotene were determined two years later. Thus, the sole function of beta-carotene in human health was considered to be its conversion into vitamin A. On the basis of observational epidemiologic studies, conducted in the mid-1970s, however, carotenoids were implicated as protective agents, first against lung cancer and then against a variety of other chronic diseases. Intervention trials employing beta-carotene, however, either have shown no preventive effect or indeed, in two cases, have enhanced the incidence of lung cancer in middle-aged male smokers and asbestos workers. The possible protective action of carotenoids can be attributed to their properties as singlet oxygen quenchers and as antioxidants, whereas their cancer-enhancing actions in lung can be ascribed to the prooxidant action of carotenoid free radicals in damaged cells. Apart from chronic diseases, beta-carotene has shown significant therapeutic value in individuals suffering from photosensitivity disorders and provides temporary relief to persons afflicted with leukoplakia. A part from a medical context, the colored carotenoids found in many living organisms and in many foods delight both the eye and the palate. Thus, human health and the enjoyment of life are greatly benefited by the presence of these interesting pigments in nature, whether or not they ultimately prove to have more specific protective effects against chronic diseases.