The impact of faults on the groundwater flow system in the Roer Valley Rift System (RVRS) is demonstrated with examples from outcrop scale to regional scale. Faults in the RVRS can form strong barriers to horizontal groundwater flow as well as enhanced vertical groundwater flow paths at the same location. The strongly anisotropic hydraulic conductivity distribution within fault zones has important implications for the modeling of groundwater flow in sedimentary aquifer systems that are cut by faults. In this study, the hydraulic behavior of fault zones is studied at different scales. An outcrop study over the Geleen Fault zone shows deformation mechanisms as particulate flow and clay smearing in great detail. Qualitative and quantitative image analysis allows for an estimate of the micro-scale variation of the hydraulic properties within a fault zone. Additional core-plug measurements indicate that the damage zone around fault zones may form preferential flow paths. On a larger scale, observations over the Peel Boundary fault near the village of Uden also indicate that vertical groundwater flow close to the fault is enhanced, which results in a discharge of the underlying aquifers at the location of the fault zone. Finally, on a regional scale, hydraulic head patterns around the lignite mining areas in Germany show the importance of faults and the variation of their hydraulic properties to regional groundwater flow patterns. © 2003 Elsevier Science B.V. All rights reserved.