The project "Plant Communities of Mecklenburg-West Pommerania and Their Vulnerability" has compiled a survey of all plant communities within this federal state in NE Germany, analysing their endangerment and evaluating their nature conservation value. This approach significantly enhances the methodology of Red Data Books. The endangerment classification has been strictly separated from the assessment of the conservation value, being derived from "Gefährdungsinhalt" (density of endangered plant species), the degree of naturalness and the responsibility for the global conservation of the plant community. The combination of endangerment and conservation value allows a definition of conservation priorities. The approach presented is characterised by clear concepts and terms, a precise scaling of categories and justified rules for the combination of criteria, clearly illustrated in matrices. Finally the paper discusses shortcomings of the approach as well as options for the application in other Red Data Books.