This publication, edited by Stéfan Arnórsson, is designed as an instructional manual of essential nuclear and complementary methodologies for a multidisciplinary approach to geothermal exploration development and monitoring. It provides comprehensive procedures for carrying out isotope and geochemical investigations of geothermal systems, i.e. sampling, analysis and data interpretation. It will be a valuable source of information for geoscientists working in geothermal, but also for those working in cold water projects, due to the similarity of methods and principles of investigations applied.