Selenium can be added to livestock diets in either inorganic forms (sodium selenite, SS), organic forms such as Se-yeast (yeast enriched in SeMet), or pure chemically synthesized Se forms such as hydroxyselenomethionine (HMSeBA), L-SeMet or ZnL-SeMet. Recent advances in Se biochemistry have provided a deeper understanding of the principal differences in absorption and metabolism between these forms.