The early interest in selenium related primarily to its toxicity, but since 1957 the element has been recognized as a dietary essential. The dietary requirement for selenium by most species is about .1 ppm. Deficiencies of selenium in cattle and sheep have been confirmed under natural grazing conditions in many countries of the world. Overt signs of inadequacy such as white muscle disease (nutritional muscular dystrophy) occur primarily in young calves or lambs born to selenium deficient dams. Infertility has increased in ewes grazing pastures low in selenium. In general, signs of deficiency have not occurred in older animals such as finishing beef cattle and lactating dairy cows. Subclinical deficiencies of selenium are not determined easily, however, and thus an inadequacy of the element may be limiting maximum animal performance under certain circumstances of drylot feeding. The current nutritional status of ruminant animals in many geographical areas and involving various feeding programs with this element has not been established. The recent widespread deficiency problems with nonruminants suggest that such an assessment should be made. Concentration of selenium in tissue, particularly in the liver, has been used in establishing selenium status of the animal. With lambs glutathione peroxidase activity in certain tissues may be a more accurate indicator of selenium adequacy than is selenium content of the tissue. Supplemental sodium selenite and sodium selenate by either oral administration or parenteral injection have prevented clinical signs of selenium deficiency and animal losses in both ruminant and nonruminant animals. Heavy pellets containing elemental selenium for placement in the rumen have proved effective. In general, organic forms of selenium are absorbed more readily by animals than are inorganic compounds. The dietary requirements for selenium and its metabolism are influenced by many nutrient interrelationships, including its interactions with sulfur, lipids, vitamin E, proteins, amino acids, and several microelements. The Food and Drug Administration gave approval in 1974 for the oral administration of supplemental selenium as either sodium selenite or sodium selenate to certain classes of swine and poultry. Similar approval in the United States for ruminants will require additional information, particularly with regard to the influence of dietary intake on concentrations of selenium in tissue and milk in beef and dairy animals.