This paper is concerned with present-day knowledge of the biological role of selenium, of its interaction with other nutrients including trace elements, and with the importance of selenium in human nutrition and health. Selenium has been shown to be an integral part of glutathione peroxidase, which catalyses the reduction of a large range of lipid hydroperoxides and hydrogen peroxide. The interrelation between vitamin E, selenium and polyunsaturated fatty acids is complex. First, selenium in glutathione peroxidase may control intracellular levels of hydrogen peroxide, which affect the formation of active oxygen metabolites that may serve as initiators of lipid peroxidation; this role of selenium is closely related to that of superoxide dismutases, which control intracellular levels of the superoxide anion. Secondly, vitamin E may control the formation of lipid hydroperoxides through its antioxidant function, as well as possibly entering into a structural relation with membrane phospholipids. Thirdly, glutathione peroxidase may catalyse the reduction of lipid hydroperoxides, formed from membrane lipids, to hydroxyacids without detriment to the cellular economy. In the field of human nutrition, the lack of selenium has been shown to be the cause of a cardiomyopathy known as Keshan disease, occurring in the People’s Republic of China. Blood selenium levels in patients from this area are compared with blood selenium levels in three other parts of the world and the conclusion is reached that the blood selenium level of populations in Keshan disease regions are exceptionally low and that Keshan disease is the first demonstration that selenium is an essential trace element for man.