The implementation of the Water Framework Directive requires a prior typology of aquatic ecosystems based on geographical features. The « Hydroecoregion» (HER) approach, previously developed in France, was extended ta Europe in the REBECCA project. This approach relies upon the concept of hierarchical ecosystem control. Primary determinants of the functioning of aquatic ecosystems are geology, relief and climate. The base data are geographicallayers describing these factors, compiled in a GIS. The common scale of the different layers is 1/1 000000. The geological map was interpreted according to the lithological structure and properties of the rocks (hardness, permeability, influence on water chemistry). A digital elevation model (DEM) with a pitch of 30 arc second (about 1km) was used ta characterize the altitude and the slopes. Hydrogeological maps were also consulted. The climatic factors refer ta precipitation (yearly average, seasonal variation) and temperature, interpreted with respect ta aquatic ecosystems functioning (water balance). The delimitation of the HER was carried out visually, seeking natural discontinuities for each of the parameters and regional concordances in their distribution. The basic principle was to minimize intra-regional variance and maximize inter-regional differences. A total of 133 HERs was identified at the most detailed level in Europe. These HER were further classified in 9 classes of relief, 21 categories combining relief and lithomorphology, and 9 climatic classes. The concepts, methodology, and the HER map, and the detailed characterisation of these HERs are presented in this report.