This book covers the distribution, hydrochemistry and geophysics of the naturally occurring stable isotopes namely: hydrogen, oxygen and radioactive tritium, carbon and other cosmogenic and radiogenic isotopes of the uranium-thorium series, in the oceans and in atmosphere, the earth's surface and ground water. The use of environmental isotopes in the three main areas of natural waters is discussed: origin, dynamics and residence time in natural reservoirs. The origin of the hydrosphere is examined in the light of isotopic, new cosmochemical and recent theoretical results. The book will be of interest to scientists and researchers who use environmental isotopes in solving scientific and practical problems in hydrology, hydrogeology, oceagraphy, meteorology, hydrogeochemistry and cosmochemistry. Lecturers, students and postgraduates in these fields will also find it useful.