Proceedings of a symposium organized in co-operation with UNESCO, Vienna, 30 March to 3 April 1987. Nuclear techniques using isotopes are precise tools for studying water resources. They can provide solutions to the problem of the origin, distribution and properties of water in a given region, especially when they are used in combination with other tools. The contents of these proceedings illustrate the fact that isotope hydrology has matured. It is reported that the development of new nuclear techniques is enabling the application of isotopes in uses other than hydrology. Contents: Thermal water studies; Groundwater dating; Hydrology of arid and semi-arid areas; Field studies with environmental isotopes; Precipitation–surface–groundwater relationships; Pollution; Artificial tracers; Sediment transport; Poster presentations.