The main purpose of interwell tracer tests in oil and geothermal reservoirs is to monitor qualitatively and quantitatively the injected fluid connections between injection and production wells and to provide important data for better understanding of the reservoir geology in order to optimize the production strategy and thereby maximize the oil recovery or thermal energy production. Most of the information given by the radiotracer tests cannot be obtained by other means. Based on the key findings of an IAEA coordinated research project in this area, this publication describes the principles and the state of the art of radiotracer techniques for interwell investigations and provides practical guidance on the design and implementation of tracer experiments as well as on the interpretation of the results.