In Part 1 of the review of experimental data concerning reactive oxygen species (ROS) production and use, their indispensable role in regulation of multiple physiologic processes is examined. Part 2 considers the unique property of reactions in which ROS participates, the generation of energy of electronic excitation (EEE). Because of incessant ROS cycling, EEE is continuously produced in aerobic organisms. It may be used as an energy activator for multiple biochemical reactions and also may function as a specific signaling factor. This approach allows the perception of the beneficial effects of hydrogen peroxide, ozone, other ROS therapy, and foods rich in antioxidants. Both insufficient production of ROS and distortions in their use may cause anomalies in coordination of ROS generation and use, resulting in the deranged operation of biochemical and physiologic processes and the development of pathology.