Free radicals and reactive oxygen species (ROS) are considered to be dangerous pathogens as they may damage key molecular constituents of cells. However this concept approach does not take into account vital functions of ROS in normal physiology. Information has emerged that a substantial share of oxygen consumed by aerobic organisms is used for ROS production and that ROS are indispensable for regulation of multiple functions of living cells. Yet, each cell is equipped with powerful means to eliminate ROS immediately. Explanations of the mechanisms of regulatory action of ROS upon a wide spectrum of biochemical and physiologic reactions and of ROS therapeutic efficacy raise serious problems in the framework of the conventional biochemical paradigm. Here data concerning ROS production and utilization are considered with an emphasis on an apparent paradox: Why does the body produce a lot of ROS and then eliminate them as soon as they appear?