This book explores the molecular mechanism of low-intensity optical radiation action on patients undergoing phototherapy, the use of which has been expanding in recent years. The effect of phototherapy on blood oxygenation, as well as on metabolic processes, is studied here using optical radiation of various wavelengths. The book evaluates changes of blood coagulation, lipid exchange and glucose content, and considers the laws of blood photomodification and the main stages of organism photoactivation. Special attention is also given to the susceptibility of individual patients to blood irradiation and the methods for its control.