This monograph explores the fundamental principles of stable isotope fractionation for hydrogen, oxygen, sulfur, carbon, nitrogen, strontium, and lithium. It synthesizes and analyzes the results of both theoretical and experimental research in the fields of hydrochemistry, isotope hydrology, oceanography, biochemistry, and ecology. A significant portion of the work is dedicated to studies conducted in the former Soviet Union, with over 40% of the content comprising the author's original research. From the perspectives of isotopy and water chemistry, the book examines the evolution of oceanic waters, associated groundwater from deep aquifers, infiltration waters, as well as marine bodies (such as the Aral, Caspian, Azov, Dead, and Red Seas) and freshwater ecosystems (including Lake Baikal and Lake Ladoga) and rivers (like the Don, Kuban, Volga, Belaya, Ufa, Selenga, and Angara). This comprehensive analysis offers valuable insights into the intricate relationships between isotopes and environmental processes, making it an essential resource for researchers and practitioners in related fields.