[en] Surface or underground water circulations and interactions are more and more often studied with the help of geochemistry and more particularly by using isotopic tracers. These isotopic tracer techniques allow, in particular, to define for each system under study, the natural or anthropic origin of the chemical elements, their behaviour, their transport in the different compartments, the circulation schemes of deep fluids and their interaction with the surrounding rocks. This article presents: 1 - the isotopes: definition, measurements and uses (stable and instable isotopes, measurement means, some examples: stable isotopes of the water molecule, boron isotopes, sulfur and oxygen isotopes of sulfates, strontium isotopes, nitrogen isotopes of nitrates); 2 - isotopes and water cycle: atmospheric tracing (rainfall signal at the drainage basin scale and at the country scale, aerosols characterization in urban areas), management of alluvial aquifers, underground waters and origin of nitrogenous contaminations, underground and surface waters in the context of aquifer floods: the case of the Somme basin, underground waters at the basin scale: heterogeneities, interactions and management processes (stable isotopes of the water molecule, S and O isotopes of dissolved sulfates, strontium isotopes); 3 - conclusion. (J.S.)