The review is devoted to the role of selenium and its compounds for living organisms. Selenium as an antioxidant protects the cells from generating free radicals. It constitutes an integral part of approximately 20 enzymes. Glutathione peroxidase, which participates in metabolism of H2O2 and plays a protective role against lipids oxidation is one of the most important enzymes containing of selenium. In organisms, selenium occurs most frequently in amino acids, where it replaces sulphur, such as selenocystein and selenometionine, as well as in bonds with proteins. Amino acid bonds are significant in functions of RNA, in biosynthesis and activity of thyroid hormones. Selenium deficiency leads to decrease in selenium-dependent glutathione peroxidase, which occurs in mitochondria and cytosole. It causes also heart muscles diseases, the skeletal and circulatory systems disorders, neoplasms, infections, and cold.