Fluorescent tracers are widely used to determine the speed of water flow in surface or subterranean media because of their easy implementation and reasonable cost. The health risks for those who drink water marked by these molecules are not, however, well known. Little information is available about the hazards associated with these products and the toxicological data are sometimes contradictory. Characterization of oral exposure, the only route considered, is complicated. While passage of the tracer peak is accompanied by acute exposure, maximum concentrations are always very low, except in the event of accidental release. Delayed release of a fraction of the tracer, trapped in the ground, may lead to subchronic exposure. Without toxicological reference values for these different molecules, no risk assessment can be conducted. To establish guidelines for the use of these products, the US EPA (Office of Toxic Substances) used the SAR (structure-activity relationship) method. The evalua-tion showed that none of the fluorescent tracers presented a substantial risk to humans or the environment at the concentrations currently used. The risk of toxicity for consumers is nonexistent if the concentration remains below 1-2 mg/L per 24 h. On the other hand, an analysis in Germany, based on genotoxicity and ecotoxicity testing, resulted in a recommendation to avoid the use of some tracers. These various findings nonetheless indicate that all of these products are usable without any particular limitations, although some molecules - such as fluorescein and stilbenes - are more innocuous than others. This does not rule out adjustment of the quantity of product injected into the tracing target and implementation of measures to limit exposure, when necessary