Water is scarce, and thus must be managed adequately in order to satisfy the global demand within a water resources integral planning. Management can act either on offer or demand. Proper management implies an accountability system capable both of reproducing the existing situation and quantifying the effect of management scenarios upon resources quantity and quality. The need for quantification makes indispensable numerical models in general, and hydrogeological models in particular. We present some of the elements that are included in a hydrogeological model, including the need for managing huge amounts of data and some technical aspects on discretization and calibration of numerical models. Hydrogeological models must be included into management models that account for conjunctive use of all available resources.