The early interest in selenium (Se) focused on its toxicity, but since 1957 it has been recognized as an essential dietary element. The diet is the major Se source and approximately 80% of dietary Se is absorbed, depending on the type of food consumed. In several regions of the world, the content of Se in diets has been estimated as insufficient. The dietary requirement for Se for most species is about 0.3 ppm. Deficiencies of Se in animals have been confirmed under natural grazing conditions in many countries. Symptoms of deficiencies such as white muscle disease occur primarily in young calves or lambs born in low Se regions and thus we need to provide sufficient amounts in animal diets. Numerous studies have demonstrated that Se plays an important role in animal health. Therefore, this review gives a brief outline of the current information on the physical, chemical and metabolic properties of Se, oxidative stress, antioxidant defense, dietary requirement, deficiency, toxicity and effects of Se on fertility and performance. پرونده مقاله