The Aquifer Reference System of France (BDLISA1) proposes a delineation of its hydrogeological units, including aquifer reservoirs and impervious units, according to local, regional and national scale. Each unit is characterized by a class of permeability, porosity and type of geological formation. The ranking number increases with increasing depth and this enables identification of the vertical position at each point with respect to the other units. Thus, an aquifer in a sedimentary basin can be covered by several dozen other units. The classes enable identification of all carbonate rocks in France and their vertical position: from surface outcrops down to deep carbonates (Figure 1). Carbonate rocks cover more than 60% (350,000 km²) of Metropolitan France, of which 55% (193,000 km²) in outcrop. Each carbonate rock is a potential karst aquifer (World Karst Aquifer Mapping Project). Nevertheless, the absence or differing degrees of karstification demand a classification of these aquifers according to karst indicators. Specialists strive to find approaches that are both adapted and homogeneous on a national scale. Initially, BDLISA was built on a regional scale, with each region employing its own approach depending on data availability: mapping of all surface disorders related to karst (swallets, sinkholes, dolines etc.), tracer tests in karst aquifers and identification of groundwater catchment areas, borehole analyses (flow out by fractures and karst)2, karst springs, wells with very high specific discharge, stream sink, etc. In a later stage, and in order to harmonize the national scale of BDLISA, two approaches were proposed depending on the surface karstification rate: 1) The whole unit is classed as a karst aquifer: four classes distinguish whether the geological formation can be characterized by a network of underground karst systems or, moreover, a second or third network, all being part of the hydrodynamic system. 2) Karst only constitutes part(s) of the unit: a supplementary layer enables the delineation of areas inside the aquifer unit where the presence of karst is well known. The national cavity database3 provides a non-exhaustive listing of natural karst cavity as well as karst springs and wells with very high specific discharge (BSS). These GIS layers were overlain on the extension of carbonate rocks. This information enabled a better understanding of many karst areas. For approach 1, the complete unit classed as karst aquifer, the mapping reveals 2/3 of the karst at outcrop and 1/3 in the subsurface (25%, corresponding to 545 units at local scale). For approach 2, 108 karst areas have been delineated and linked to their 60 aquifer units. A new 1:1M scale hydrogeological map of France will be published in 2015, based on the information of the BDLISA units. It is designed for hydrogeologists and is a communication and educational tool: the representation of karst aquifers had to be modified in order to allow visibility at this scale by a differentiation of karst aquifer outcrops and deeper karst units, the display of karst springs (World Karst Aquifer Mapping Project) and karst aquifer labels.