In heterogeneous porous aquifers, simulations and predictions of groundwater flow and solute (contaminant) transport require detailed knowledge of aquifer parameters and their spatial distribution. In most cases this information cannot easily be obtained at acceptable expenses. In general, subsurface investigation techniques are applied only at borehole locations, and the parameter values measured have to be regionalized in order to obtain continuous parameter fields. Geophysical measurements very often yield unsatisfactory results due to resolution, detection range and parameterisation problems. In such situations tracer tests offer the possibility to efficiently investigate the aquifer between the wells and to characterize the relevant aquifer properties based on effective parameter values. Tracer tests can be performed at laboratory and field-scales with depth integrated (two-dimensional) or multilevel (three-dimensional) set-ups, and under natural or forced hydraulic gradient conditions. Both non-reactive and reactive tracer compounds can be used. This contribution covers and gives examples on the following topics: depth integrated and three-dimensional natural and forced gradient tracer test methods together with their fields of application at different transport scales, novel tracer compounds and applications, high resolution multileve-multitracer methods and high resolution multileve-multitracer equipment, as well as approaches to evaluate tracer experiments and to quantify tracer transport. In this way the paper shows some recent trends in tracer based subsurface investigation and emphasizes the advantages and importance of modern tracer testing. Keywords: Groundwater; Solute transport; Subsurface investigation; Tracer testing; Stochastic modelling
