The development of a Pan-European Ecological Network is nowwidely recognised as an important policy initiative in support of protected Natura 2000 sites. The site selection is based on habitats as defined in the Annex I of the Habitats Directive. Whilst there is information about the presence of these habitats in Natura 2000 sites, there is no detail of their distribution elsewhere in Europe. The present paper describes amethodology that identifies the spatial distribution of habitats across Europe so that their actual extent can be determined. Five methodological steps are involved starting with selection of appropriate spatial data sets, defining knowledge rules from the descriptions of Annex I habitats, continuing with additional
ecological expert knowledge when needed, implementation of the models, and finally the validation.
Spatial distribution models were derived for 27 habitats representing the most significant ecosystems. This spatial modelling approach is illustrated with one detailed example. Validation showed that mapping accuracy depends on the habitat description available but also upon its spatial character. Thus widespread habitats such as forests were accurately assessed whereas dispersed classes such as freshwater systems were more difficult to assess. Possible methodological improvements are suggested, such as inclusion
of vegetation relevés to improve the knowledge rules. Extension of the methodology to other habitats would require a moderate effort since data collection and processing has nowbeencompleted and it is this which is the most time consuming part of the process. We conclude that our method maps widespread European habitats with unprecedented accuracy.