Selenium, an essential nutrient in a human diet, can be administered in both organic and inorganic forms. Organic forms are easier to ingested than inorganic ones. The biological functions of selenium are mediated largely by selenoproteins. Selenium is present in mammalian selenoproteins as selenocysteine. Human selenoproteins encoded by 25 genes are involved in glutathione- dependent hydroperoxide removal, reduction of thioredoxins, selenophosphate synthesis, activation and inactivation of thyroid hormones, repair of oxidized methionine residues, and ER-associated protein degradation. These functions are responsible for the role of selenium in human health, including its pro- and anticancer activities, roles in the immune system, and other functions. Selenium is regarded as a controversial trace element. Its deficiency is associated with cancer, cardiovascular diseases, infertility, thyroid diseases and poor immune functions. On the other hand, selenium in high levels has been considered as a poison, causing symptoms such as memory loss, fatigue, diarrhoea and vomiting, in addition to which it can increase the risk of type 2 diabetes and cancer. There are many aspects of the metabolism of selenium and selenoproteins that remain to be investigated. The purpose of this article is to bring up to date the current status of the developing field of selenium research, centered around the health benefits attributed to this element. The relationship between the selenium status in a human organism and selected health outcomes is discussed.