Selenium is a trace element, essential in small amounts, but can be toxic in larger amounts. Levels in the body are mainly dependent on the amount of selenium in the diet, which is a function of the selenium content of the soil. Humans and animals require selenium for normal function of more than about 30 known selenoproteins, of which approximately 15 have been purified to allow characterisation of their biological functions. The selenoproteins are comprised of four glutathione peroxidases, three iodothyronine deiodinases, three thioredoxin reductases, selenoprotein P, selenoprotein W and selenophosphate synthetase. Selenium is essential for normal functioning of the immune system and thyroid gland, making selenium an essential element for normal development, growth, metabolism, and defense of the body. Supportive function of selenium in health and disease (male infertility, viral infections, including HIV, cancer, cardiovascular and autoimmune diseases) is documented in great number of clinical examinations. A great number of studies confirm that selenium supplementation play a preventive and therapeutical role in different diseases. Definitive evidence regarding the preventive and therapeutical role of selenium as well as the exact mechanism of its action should be investigated in further studies. Investigations in Croatia indicate a possibility of inadequate selenium status of people in the area.