In this paper, we demonstrated that the bog-fen boundary is clearly determinable not by pH, but by a set of nutrient-requiring species that avoid truly ombrotrophic conditions in central Europe. We therefore defined fens as groundwater-fed wetlands that host low productive nutrient-limited vegetation dominated by Cyperaceae and bryophytes. The fertility gradient within fens is easily distinguishable using both plant and animal data, but it appears primarily within calcium-richer fens. We suggest defining fen grasslands by the high abundance of nutrient-requiring grasses and forbs rather than purely by fen origin and management. The ecological differentiation of all proposed fen types was tested using a data set from two different regions. Both conductivity and pH differ significantly between pairs of vegetation types. All proposed fen types also markedly differ in molluscan assemblages.