This review is aimed at highlighting the role of selenium (Se) in enhancing the antioxidant status in the blood, immune response, reproductive performance, rumen fermentation, as well as feed efficiency, and weight gain in small ruminants. Selenium is supplied in small ruminant’s diet in organic form, inorganic form, or selenium nanoparticles (nano-Se). Each form of Se is metabolized differently and exhibits different levels of bioavailability with nano-Se having the highest bioavailability followed by organic Se. A sufficient supply of Se in small ruminant’s diet is important to maintain the proper functioning of the antioxidant defense system which reduces the detrimental effects of free radicals in cells thereby preventing the incidence of metabolic diseases. The ability of Se to improve the antioxidant capacity of the cells also helps to strengthen the immune response of animals and ameliorate the detrimental effects of heat stress. Selenium has also been proven to modify rumen fermentation and microbiota which translated to improved feed efficiency and enhanced weight gain. However, the activities and bioavailability of Se are limited by several factors including the rumen microbiota, the form of Se, and competition with other minerals. Further studies need to investigate the effects of Se on rumen microbiology and heat stress in small ruminants. پرونده مقاله