[en] Full text: The principles of groundwater dating up to about 40 000 yr by means of ¹⁴C were elaborated about fifty years ago. Prior this time hydrologic estimates of the recharge rate and flow velocities of groundwater were poor. Already the first ¹⁴C dates proved that any elderly water budget or geohydraulic concept had to be substantially revised especially in arid zones. Groundwater is usually considerably older than expected before the introduction of the ¹⁴C method. During one decade of gathering experience with groundwater dating it was recognized that absolute dates of groundwater require a so-called reservoir correction either based on the carbon isotope or hydrochemical compositions of the inorganic carbon species or both. A variety of simple to complex correction models for conventional ¹⁴C dates was elaborated besides more easily applicable empirical correction methods. Attempts to date groundwater with ¹⁴C in the dissolved organic constituents elucidated other kinds of hydrological problems. The requirement of absolute dates is primarily restricted, however, to scientific-relevant paleohydrological studies. In applied hydrology geohydraulic estimates of the budget of fresh groundwater directly profit from easily determinable relative groundwater ages. Such ¹⁴C dates allow estimates of regional geohydraulic parameters of the aquifer and its over and underlain aquitards, monitoring of the groundwater movement and detection of overexploitation. In any case the interpretation of groundwater ages requires a good understanding of the hydrodynamics of the system to be dated beside that of the hydrochemical composition. In arid and semi-arid zones, these dates allow to differentiate between renewable and non-renewable groundwater resources. This information is indispensable for numerical modeling as neglecting of non-stationary recharge conditions results in overestimated recharge rates. The difficulty to determine absolute ¹⁴C dates of groundwater has supported manifold efforts to search for other environmental isotopes applicable for water age determination. ³⁶Cl has been a promising isotope to date fresh water resources with ages between 100 000 and more than 1.5 million years. However, there are several severe problems in the interpretation of ³⁶Cl dates. One is underground production during groundwater aging and the other is the usually high mineralisation and chloride concentration of old groundwater which lower drastically the detection limit. Similar problems obstruct groundwater dating by ¹²⁹I theoretically within 3 to 90 million years. Isotope hydrologists hope to simplify the detection of ⁸¹Kr. This method namely dates also mineralized groundwater with ages between 50 000 up to 1 million years. At present the analytic and detection requirements are, however, still too complicated for its wide use in applied hydrology. The uranium isotope compositions allow dating within the oxygenated zone of fresh groundwater resources as successfully proved for limestone and sandstone aquifers. The situation in the reduction zone is theoretically well evaluated but the complexity of processes as absorption and formation of chemical complex compounds has still prevented any successful application. Theoretically, the continuos formation of ³He and ⁴He as well as ⁴⁰Ar in aquifers allows groundwater dating between 10 000 and 10 million years. Diffusion loss, complex exhaustion of such gases from both the earth mantle and crust prevents, however, reliable datings of groundwater. Age determination of old groundwater challenges modelers and isotope hydrologists to find a linkage between the results of finite element modeling and isotope hydrological studies. Even the basic principles are not yet elaborated. On one side a groundwater sample to be dated represents a relatively large compartment of unknown in size of the groundwater resource. On the other side, this compartment contains an unknown number of finite elements of the model. Therefore, the modeled ages of single elements cannot be simply linked to isotope dates. Success is wanted as isotope dates are most suited to calibrate and validate numerical models. IAEA conferences have been an ideal forum for the sometimes explosive and enthusiastic development and improvement of the various dating methods of groundwater. Controversial discussions sharpened and opened the understanding for a reliable hydrology-related interpretation of groundwater dates. These conferences set benchmarks for the various methods. Moreover, it was the IAEA support in training and application of these methods, in setting up laboratories round the world which formed the solid platform for the world-wide successful introduction of these methods in the management of fresh groundwater resources and the protection measures against pollution. (author)