A swamp is a wetland that is forested. Many swamps occur along large rivers, where they are critically dependent upon natural water level fluctuations. Other swamps occur on the shores of large lakes. Some swamps have hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp forests and "transitional" or shrub swamps. The water of a swamp may be fresh water, brackish water or seawater. Some of the worlds largest swamps are found along major rivers such as the Amazon, the Mississippi, and the Congo.

Swamps are characterized by very slow-moving waters. They are usually associated with adjacent rivers or lakes. In some cases, rivers become swamps for a distance. Swamps are features of areas with very low topographic relief.

Swamps were historically drained to provide additional land for agriculture, and to reduce the threat of diseases born by swamp insects and similar animals. Many swamps were also heavily logged, which also required construction of drainage ditches and canals. These contributed as well to drainage, and, along the coast, allowed salt water intrusion that converted swamps to marsh or open water. Large areas of swamp were therefore lost, or degraded. Louisiana provides a classic example of wetland loss from these combined factors. Europe has probably lost nearly half its wetlands. As another example, New Zealand has lost 90 percent of its wetlands over the past 150 years. It is now appreciated that swamps provide valuable ecological services including flood control, fish production, water purification, carbon storage, and wildlife habitat. In many parts of the world swamps are protected. In parts of Europe and North America, swamp restoration is becoming widespread. Often the simplest steps to restoring swamps are to plug drainage ditches and remove levees.