The hydrogeological role of faults is a key area of uncertainty in understanding the movement of groundwater in the vicinity of a deep geological repository for radioactive waste. Evaluations of regional flow fields, and predictions of radionuclide migration rates and paths through the geosphere, require an understanding of fluid flow paths along faults as well as the effects of fault movement. The European Commission, under the Nuclear Fission Safety Research and Training Programme (1994-1998), is supporting a project aimed at evaluating the level of detail at which fault hydrogeology and the hydrodynamics of fault movement should be included in performance assessments of radioactive waste disposal systems. The Environment Agency of England and Wales, United Kingdom Nirex Limited, ANDRA, POSIVA, and SKI are also sponsoring the research. The project has involved the development of a detailed three-dimensional model of fluid flow and radionuclide transport within a fault zone, incorporating a range of hydrogeological structures identified through a review of observations of fault hydrogeology. Also, a model of rook deformation and associated fluid pressure perturbations has been developed in order to evaluate the changes in groundwater flow associated with fault reactivation. Permeability along a fault depends on the amount and type of fault rock present (e.g., breccia and gouge zones in the fault core), the density of structures in the damage zone around the fault (e.g., fractures and veins), and the extent to which water-rock reactions have modified these subsequent to their formation. Steps and bends develop in fault zones where independent faults, or related en echelon fault surfaces, overlap. Relay zones, characterized by stress concentrations and induced fracturing, link the faults. Thus, fault zones typically consist of arrays of interrelated, offset fault segments, which form fully three-dimensional hydrogeological structures. The three-dimensional fracture network code, FRACAS [1], has been modified so that observed hydrogeological features of faults can be modelled. Fault zone reactivation could affect groundwater flow conditions. Repositories sited in regions that are expected to be subject to future periods of glacial loading and unloading could be affected by the accompanying changes in stress and strain. Induced motion along faults could result in groundwater movement towards the zone of active shearing. The rock mechanics code, FLAG [2], is being used to demonstrate the contrasting effects of fault movement on the stress distribution within and outside a fault zone. A fluid flow algorithm, which includes a dilational term to account for the plastic behaviour of the fault rock, has been developed to assess the effects of fault movement on groundwater flow. Simulations and sensitivity analyses are being undertaken using the FRACAS and FLAG codes to assess the consequences of fault heterogeneity and the hydrodynamics of fault reactivation on groundwater transport. The project is aiming to develop constitutive expressions for flow and transport in fault zones that are computationally tractable for use in performance assessment models.