Environmental isotope and nuclear techniques provide unmatched insights into the processes governing the water cycle and its variability under past and present climates. This monograph is recommended to advanced students and specialists and presents historical perspective, state of the art applications and new developments of isotopes in hydrology, environmental disciplines and climate change studies. The spectrum of isotope applications addressed in this monograph ranges from the assessment of groundwater resources in terms of recharge and flow regime, identification of palaeogroundwater, water balance of river basins and lakes, to studies of the past and present global environmental and climate changes. The contributions are written by renowned specialists in the various application fields.