The first part of the presentation will be a review of fundamentals relating to stable O, H and S isotopes and tritium. In the second part, some case studies of general importance will be discussed, including: 1. O and H isotopes as a groundwater dating tool in river basins with dams; 2. isotopes as indicators of the importance of paleogeography in controlling groundwater flow; 3. use of tritium since the disappearance of the bomb spike; 4. use and abuse of isotope data for precipitation.