Groundwater dating is known to be determination of the residence time of groundwater. This residence time is commonly defined as the length of time the water has been isolated from the atmosphere. The paper reviews isotope methods available for dating of old groundwater. Their residence time is supposed to be far outside the range of C-14 dating which is about 50000 years. These dating methods are based on the radioactive decay of long-lived radionuclides produced by interaction of cosmic radiation with gases in the atmosphere (Kr-81, Cl-36, I-129), on the accumulation of noble gases suo-surface produced by nuclear reactions including radioactive decay of U/Th series radionuclides (He-4, He-3), and on the disequilibrium between uranium isotopes due to water-rock interaction and radioactive decay (U-234, U-238). The paper presents the principles of these dating methods, refers to recent case studies and summarizes problem areas for further development and application of these methods.