This paper gives some introductory information on the use of wetlands for wastewater treatment. It focuses mainly on the functioning of constructed wetlands, in particular surface-flow and infiltration wetlands. The various processes which lead to water purification are briefly explained, in relation to the factors which influence their efficiency. The possibilities for optimization of the design and management of such systems are illustrated with data on the functioning of a wastewater infiltration wetland in The Netherlands. In general, constructed wetlands can be designed to remove more than 90% of BOD, COD, suspended solids and bacteriological pollution from the through-flowing wastewater. Removal of N and P remains, however closer to 50% in most cases.