Wetlands are characterized by the occurrence of plants that are adapted to and grow only on waterlogged or shallow inundated soils. They occur in all climatic zones, from boreal to tropical, and include bogs, fens, herbaceous marshes, woody swamps, shallow water bodies, riverine floodplains, and coastal beaches. Wetlands, other than bogs, are highly productive systems and support globally large biodiversity (from microorganisms to mammals) disproportionate to their total area on the Earth’s surface. Wetlands provide many goods such as food, fiber, fuel and timber, and services such as groundwater recharge, flood abatement, habitat for numerous rare, endangered and threatened as well as migratory species, improvement of water quality, and enhancement of the esthetics of the landscape. Wetlands also influence climate change through carbon sequestration and methane emission. Wetlands are often likened to the kidneys of the Earth because various nutrients and pollutants are removed from the storm water, agricultural runoff, and other wastewaters passing through them by several physical, chemical, and biological processes of which the settlement of particulates, adsorption of phosphorus, microbial mineralization, denitrification, and plant uptake play major roles. This potential for water-quality improvement is now exploited by designing constructed wetlands for the treatment of different kinds of wastewaters.