A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetland along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. The unique water chemistry of fens is a result of the ground or surface water input. Typically, this input results in higher mineral concentrations and a more basic pH than found in bogs. As peat accumulates in a fen, groundwater input can be reduced or cut off, making the fen ombrotrophic rather than minerotrophic. In this way, fens can become more acidic and transition to bogs over time.

Fens can be found around the world, but the vast majority are located at the mid to high latitudes of the Northern Hemisphere. They are dominated by sedges and mosses, particularly graminoids that may be rarely found elsewhere, such as the sedge species Carex exilis. Fens are highly biodiverse ecosystems and often serve as habitats for endangered or rare species, with species composition changing with water chemistry. They also play important roles in the cycling of nutrients such as carbon, nitrogen, and phosphorus due to the lack of oxygen (anaerobic conditions) in waterlogged organic fen soils.

Fens have historically been converted to agricultural land. Aside from such conversion, fens face a number of other threats, including peat cutting, pollution, invasive species, and nearby disturbances that lower the water table in the fen, such as quarrying. Interrupting the flow of mineral-rich water into a fen changes the water chemistry, which can alter species richness and dry out the peat. Drier peat is more easily decomposed and can even burn.

Fens are distributed around the world, but are most frequently found at the mid-high latitudes of the Northern Hemisphere. They are found throughout the temperate zone and boreal regions, but are also present in tundra and in specific environmental conditions in other regions around the world. In the United States, fens are most common in the Midwest and Northeast, but can be found across the country. In Canada, fens are most frequent in the lowlands near Hudson Bay and James Bay, but can also be found across the country. Fens are also spread across the northern latitudes of Eurasia, including Britain and Ireland, as well as Japan, but east-central Europe is especially rich in fens. Further south, fens are much rarer, but do exist under specific conditions. In Africa, fens have been found in the Okavango Delta in Botswana and the highland slopes in Lesotho. Fens can also be found at the colder latitudes of the Southern Hemisphere. They are found in New Zealand and southwest Argentina, but the extent is much less than that of the northern latitudes. Locally, fens are most often found at the intersection of terrestrial and aquatic ecosystems, such as the headwaters of streams and rivers.

It is estimated that there are approximately 1.1 million square kilometers of fens worldwide, but quantifying the extent of fens is difficult. Because wetland definitions vary regionally, not all countries define fens the same way. In addition, wetland data is not always available or of high quality. Fens are also difficult to rigidly delineate and measure, as they are located between terrestrial and aquatic ecosystems.

Rigidly defining types of wetlands, including fens, is difficult for a number of reasons. First, wetlands are diverse and varied ecosystems that are not easily categorized according to inflexible definitions. They are often described as a transition between terrestrial and aquatic ecosystems with characteristics of both. This makes it difficult to delineate the exact extent of a wetland. Second, terms used to describe wetland types vary greatly by region. The term bayou, for example, describes a type of wetland, but its use is generally limited to the southern United States. Third, different languages use different terms to describe types of wetlands. For instance, in Russian, there is no equivalent word for the term swamp as it is typically used in North America. The result is a large number of wetland classification systems that each define wetlands and wetland types in their own way. However, many classification systems include four broad categories that most wetlands fall into: marsh, swamp, bog, and fen. While classification systems differ on the exact criteria that define a fen, there are common characteristics that describe fens generally and imprecisely. A general definition provided by the textbook Wetlands describes a fen as "a peat-accumulating wetland that receives some drainage from surrounding mineral soil and usually supports marsh like vegetation."

Three examples are presented below to illustrate more specific definitions for the term fen.

In the Canadian Wetland Classification System, fens are defined by six characteristics: