Selenium is an essential trace element and integral part of many antioxidant enzymes such as glutathione peroxidase and selenoprotein P in humans and animals. Deficiency of selenium leads to various clinical consequences including cancer, cardiovascular diseases, type 2 diabetes and lung disorders. This review gives a brief outline of the current information on selenium in the environment, its natural sources, dietary requirement, various selenoproteins, the role of selenium as an antioxidant in defense systems, as well as its antimicrobial and radioprotective abilities. The relationship between selenium deficiency and various health outcomes, in particularly cardiovascular disease, nervous and gastrointestinal abnormalities dysfunction of the thyroid and immune systems type 2 diabetes and fertility, are also reviewed. The exact chemical form and dose, which results in the normal functioning of numerous body systems or risk of disease are intricate but need to be elucidated through good clinical practice for efficient public health strategies.