The application of natural isotopes, stable as well as radioactive, has become a widespread tool for hydrological research, especially surface- and groundwater behaviour and exploration. By far the most common isotopes applied are those of the elements of hydrogen, oxygen and carbon, crucial in the water cycle. The concentration ratios of the stable isotopes in water vary, depending on the source of the water and natural processes such as evaporation and condensation. Radioactive isotopes may provide indications,depending on natural conditions, of the source of water and age, i.e. the time elapsed since infiltration. This is a textbook, explaining in a simple, but mathematically, physically and chemically correct form, the consequences of natural processes to stable isotope concentrations and radioactivities. After a broadly theoretical introduction, the elements of the water cycle are treated successively: Precipitation: globally, regionally and locally; Different forms of surface water: rivers, rivulets and small streams, lakes and stagnant waters, estuaries and the sea; Groundwater: infiltration and groundwater flow. The book is intended for students in the Earth Sciences, especially in Hydrology; trainees from developing countries; laboratory technicians and assistants; engineers involved in (geo)hydrological fieldwork and anyone who starts his work in natural abundances of stable or radioactive isotopes.