Selenium occurs as inorganic selenite or selenate and in organic forms in plants and other organisms used for food. The human selenoproteome consists of 25 selenoproteins. The main groups are glutathione peroxidases 1-5, iodothyronine deiodinases 1-3, thioredoxin reductases, selenoprotein P (SelP), and other proteins mostly with unknown function. In selenoproteins selenium occurs as selenocysteine. SelP works as a transporter of selenium between the liver and other organs. Selenium in the form of selenomethionine can also unspecifically substitute for methionine in other proteins. No specific deficiency condition has been described in humans. The aetiology of Keshan disease, a cardiomyopathy, is a combination of coxsackie virus and low selenium. Selenium status has been linked to the incidence of cancer and other diseases. Excess selenium can produce selenosis in humans affecting liver, skin, nails and hair. Recommended intake and upper tolerable level are 40-55 and 300 microg/day. A better chemical characterization of selenium compounds in foods and in particular supplements as well as knowledge on the apparent differences in biological activity between selenium compounds, both with respect to nutrition, disease protection and adverse effects, are needed. Supplementation studies should in addition to possible beneficial effects also focus on the possibility of possible adverse effects.