Tracers are tools used in environmental research both to make processes more visible and to discover unknown processes. Tracers were used, for example in marine research, to answer the question about where the Gulf Stream ends. EAWAG is investigating similar questions about streams and lakes by using isotope tracers. Trace compounds in environmental repositories can be used to reconstruct changes in climatic conditions. A relatively recent development is the use of indicators of biological processes; biomarkers allow us to assess the effects of various pollutants on organisms.