A fen is one of the six main types of wetland and one of two types of mire. It is usually fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is pH neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients. They are usually dominated by grasses and sedges, and typically have brown mosses in general including Scorpidium or Drepanocladus. Fens frequently have a high diversity of other plant species including carnivorous plants such as Pinguicula. They may also occur along large lakes and rivers where seasonal changes in water level maintain wet soils with few woody plants. The distribution of individual species of fen plants is often closely connected to water regimes and nutrient concentrations.
Fens have a characteristic set of plant species, which sometimes provide the best indicators of environmental conditions. For example, fen indicator species in New York State include Carex flava, Cladium mariscoides, Potentilla fruticosa, Pogonia ophioglossoides and Parnassia glauca.
Fens are distinguished from bogs, which are acidic, low in minerals, and usually dominated by sedges and shrubs, along with abundant mosses in the genus Sphagnum.