Constructed wetlands are now used to improve the quality of point and nonpoint sources of water pollution, including storm water runoff, domestic wastewater, agricultural wastewater, and coal mine drainage. Constructed wetlands are also being used to treat petroleum refinery wastes, compost and landfill leachates, fish pond discharges, and pretreated industrial wastewaters, such as those from pulp and paper mills, textile mills, and seafood processing. For some wastewaters, constructed wetlands are the sole treatment; for others, they are one component in a sequence of treatment processes. One of the most common applications of constructed wetlands has been the treatment of primary or secondary domestic sewage effluent. Constructed wetland systems modelled after those for domestic wastewater are being used to treat the high organic loads associated with agriculture. The treatment of wastewater or storm water by constructed wetlands can be a low-cost, low energy process requiring minimal operational attention. As a result of both extensive research and practical application, insight is being gained into the design, performance, operation, and maintenance of constructed wetlands for water quality improvement. Constructed wetlands can be sturdy, effective systems. However, to be effective, they must be carefully designed, constructed, operated, and maintained.