A pan-European COST action (co-operation in science and technology), including delegates from 16 countries, worked from 1997 to 2002 to develop an approach to vulnerability and risk mapping that took into account the particular characteristics and the known high vulnerability to contamination of carbonate (karst) aquifers. The major task of the COST Action was to develop a general, non-prescriptive approach to intrinsic vulnerability mapping, which could be adapted into methods appropriate for use in individual karst areas of Europe. The European Approach uses four factors in assessing intrinsic vulnerability: Overlying layers (0), Concentration of flow (C), Precipitation regime (P) and Karstic network development (K). The factors 0, C and K represent the internal characteristics of the system, while the P factor is an external stress applied to the system. For resource vulnerability mapping, the factors 0, C and P should be taken into consideration, while, in addition, the K factor should be taken into account for source vulnerability mapping. A final aim in developing the European Approach was that the approach, though karst sensitive, should not be completely karst centred to the extent that it could not be used in other groundwater environments. This aim has been realised to the extent that the P and 0 factors have universal applicability in assessing vulnerability, whilst the C and K factors relate to the particular characteristics of karst aquifer systems,