Since the discovery of selenium (Se) as a component of glutathione peroxidase (GSH-Px), there have been a number of seleno-compounds identified in animal tissue. Although the biological functions of Se in different compounds have not been clearly elucidated, it is becoming clearer that Se plays an important role in initially assigned the cytosolic antioxidant function and several physiological metabolisms in animal`s body. As Se toxicities or deficiencies have been demonstrated in animals under practical conditions, concerns have been expressed about the possible impact on human health of either excessive Se exposure or deficient Se intake by animal. Because inorganic and organic Se have in many respects different bioavailability and biological activity, it is very important to figure out the characteristics of Se, differences and similarities between inorganic and organic Se sources in the body metabolism. There is an urgent need for research work to find optimal supplemental levels of Se in different animal species for food safety of human being as well as for animal`s normal growth. Consequently, this review is intended to introduce the importance of Se, deficiency and toxicity with comparison the metabolic pathways of Se in inorganic and organic forms.