The research for radioactive waste disposal involves detailed studies of the hydrogeological system at various scales. The regional groundwater model is the largest-scale model, providing boundary conditions for smaller-scale flow and transport models and for long-term predictions.The major challenges for its newest update were as follows: to better understand regional groundwater flow, to better calibrate the regional hydraulic parameters of the Boom Clay and the underlying aquifers, to apply new data from the site investigation, and to simulate the transient behaviour of the overexploited aquifers underlying the Boom Clay. The new regional groundwater model includes an updated geometry, new estimates of parameter values and a new conceptualisation of the hydrogeological system. Two steady states are simulated: the hypothetical original state and the equilibrium state of the aquifers assuming continuation of the present day pumping. The partly observed period of the transition between these states is simulated using a specific transient model. The information included in the current set of observations is not sufficient to estimate the hydraulic properties of the major regional aquitard (the Boom Clay) using inverse modelling. However, low value of its hydraulic conductivity, based on large number of measurements, appears to be compatible with the model.