Selenium is a non-metal, a trace element in the living world, and its greatest importance was shown in the study of selenoproteins - proteins which in their primary structure contain the amino acid cysteine with a selenium atom instead of sulphur. The main purpose of the thesis is to show the importance of incorporating selenium into cysteine and how selenium in an amino acid enables proteins to perform various life and physiological functions. Knowledge of this thesis about selenoproteins is based on scientific articles that describe how they are synthesized, what types exist and what their main functions are and what is their importance in other organisms and in the development of diseases. By reviewing scientific articles on the topic of selenoproteins, this paper tries to cover and summarize the importance of selenoproteins in everyday life. The paper uses the methods of induction and deduction. The results were achieved by the mentioned methods and the facts about selenium and selenoproteins were completed by data analysis. The general conclusion of the paper is to encourage the importance of selenium, because regardless of the fact that selenium is a trace element, its importance in biology in the form of selenoproteins cannot be ignored. Selenium deficiency, and thus selenoprotein dysfunction, leads to various disorders in the body, which is why it is necessary to focus on this area of research.