ISelenium was believed to be toxic to animals, however, in 1957, selenium was reported as a dietary vital nutrient. Selenium is available in inorganic and organic forms. In 1974, the Food and Drug Administration (FDA) regulated the supplementation of selenium in poultry diets. In 1994, the National Research Council recognized selenium as a dietary essential nutrient for laying hens. The maximum allowed selenium addition level is 0.30 mg/kg. One of the most common supplements used is sodium selenite (SS), the inorganic selenium source. However, in 2000, the FDA approved the use of the organic source of selenium, Se-enriched yeast (SY) in poultry diets. Selenium has valuable effects on animal immune status, growth parameters, production and reproduction. Experimentally, it has been indicated that SY benefits more than SS due its more bioavailability. Selenium inclusion in food-animal diets has an extra nutritional advantage to human consumers of Se enriched food-animal products.