Free radicals as extremely reactive chemical species represent a potential danger to all cells and contribute to the development of cardiovascular, neurodegenerative and inflammatory diseases, tumors, diabetes and other pathological conditions. However, at the same time, free radicals and other reactive oxygen and nitrogen species (ROS, "reactive oxygen species", RNS, "reactive nitrogen species"), by participating in phagocytosis and oxygenation, have a significant positive effect on metabolism. Within the framework of the course, the modes of action and effects of free radicals and ROS and RNS molecules in oxidative stress and the pathogenesis of various diseases will be discussed. In addition, students will be able to explain how the organism is protected from their harmful effects.