The term fen has been variously used by peatland ecologists, ground-water hydrologists, and vegetation scientists. The common denominator among all types of fens is recognition of the importance of ground-water discharge, especially mineral-rich ground water, in determining fen hydrology, chemistry, and vegetation, in contrast to wetlands whose characteristics are determined primarily by precipitation or surface-water inputs. Thus, fens tend to occur where climate and hydrogeologic setting sustain flows to the plant-rooting zone of mineral-rich ground water. In the United States, these areas include the glaciated Midwest and Northeast, as well as portions of the Appalachian Mountains and mountainous West. Individually and collectively, fens are among the most floristically diverse of all wetland types, supporting a large number of rare and uncommon bryophytes and vascular plant species, as well as uncommon animals including mammals, reptiles, land snails, butterflies, skippers, and dragonflies. Several species listed under the federal Endangered Species Act inhabit or use fens. Fens also help maintain stream water quality through denitrification and phosphorus sorption. Few estimates of loss and current extent exist, but where estimates are available, they indicate extensive loss, fragmentation, and degradation. Cultural eutrophication threatens the biological and functional integrity of remaining fens because, along with mineral-rich water, low availability of nitrogen and phosphorus controls many of their distinctive characteristics. Because they occur where ground water discharges to the surface, fens are isolated from neither ground water nor surface water. However, the majority of fens develop in headwater areas and could be defined as “isolated” for jurisdictional purposes because of their distance from navigable-in-fact waters. If so defined, the critical roles that fens play in maintaining biological diversity and stream water quality are at risk regarding federal jurisdiction over “isolated waters” because of the 2001 U.S. Supreme Court ruling in the case of Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers.