The book is a quantitative treatment of the theory and natural variations of light stable isotopes, and includes more than 100 original applications. Isotope distribution is rigorously discussed in the context of fractionation processes, thermodynamics, mass conservation, exchange kinetics and diffusion theory. The theoretical principles are illustrated with natural examples, emphasizing oygen and hydrogen isotope variations in natural waters, terrestrial and extraterrestrial rocks, and hydrothermal systems. New data on meteoric precipitation, rivers, and hydrothermal systems are included.