Tracer methods in hydrology use spatial and temporal patterns of concentrations, abundances and physical properties of natural and man-made substances for a comprehensive understanding of the cycling of matter through the hydrosphere. As such, the inherently interdisciplinary tracer approach is applicable to all compartments of the hydrological cycle and gives insights into the processes that govern fluxes of water, solutes and solids. Changes in tracer signatures provide timescales for environmental processes and reflect environmental shifts and events. The potential of the tracer approach goes well beyond the technical applications such as groundwater or sediment dating and is potentially relevant to all thematic areas of the UPH, including the coupled human-water systems. For example, agricultural activities, at a catchment scale, affect fluxes of water and lead to the release of a variety of potential tracers, many of them being contaminants. These alterations are reflected in tracer signatures of soil water, groundwater, surface water bodies, soils and sediments. Furthermore, the isotopic signatures in precipitation and runoff provide another dimension for the identification of the patterns and functions in catchment behaviour. At the same time, many hydrological systems remain poorly characterized with respect to even basic tracer characteristics, such as stable isotope composition of water. The aim of this session is to develop tracer methods and to identify potential tracer applications in the context of the 23 UPH. Contributions should address any of the specific questions of the 23 UPH and demonstrate how tracers can add information on solving it.