The monograph contains results of the coordinated research project conducted by the International Atomic Energy Agency with the purpose to explore the use of new isotope techniques together with conventional ones to better assess the hydrogeological conditions in the geological settings of nuclear power plant sites and to provide basic information that may be used for predicting the fate of radioactive contaminants, such as tritium, released into groundwater in case of a radiological accident and/or during normal operations. To this end, research projects were conducted in ten countries to characterize the groundwater system in the vicinity of NPPs which are at different stages of development. The monograph contains the reports of these ten research projects and a summary of the achievements of the individual projects. The results of the determination of isotopes 18O, 2H, 13C, 14C, 3H, 3Hе, 4Hе and noble gases in water were used. The results of traditional hydrochemical analyzes and distributions of 90Sr, uranium, and transuranic elements in groundwater were also used. This CRP aims to develop guidelines for studying the hydrogeological characteristics of groundwater in local and regional groundwater systems in the vicinity of nuclear power plants using environmental isotopes and conventional techniques. For scientists and engineers working in the field of hydrogeology, radioecology, and safety in nuclear energy, as well as for students and graduate students of geological and physical faculties of universities.