The results of experimental and theoretical studies of isotopic composition of natural waters of the Earth’s hydrosphere are analyzed and summarized in this book. These studies represent a modern direction of science of the natural waters actively developed in hydrology, hydrogeology, oceanography, and climatology starting from the middle of last century. The book is a further development and improvement of the earlier published author’s works: Cosmogenic Isotopes of the Hydrosphere, Nauka, Moscow, 1984; Isotopy of the Hydrosphere, Nauka, Moscow, 1983; Environmental Isotopes in the Hydrosphere, Nedra, Moscow, 1975 and Environmental Isotopes in the Hydrosphere, Wiley, Chichester–New York, 1982.