The use of wetlands for treating wastewater is probably the best example of ecological engineering because the mix of ecology and engineering is nearly even. The idea is to use an ecosystem type (wetlands) to address a specific human need that ordinarily requires a great deal of engineering (wastewater treatment). This application of ecological engineering emerged in the early 1970s from a number of experimental trials and is today a growing industry based on a tremendous amount of experience as reflected by a large published literature. Although there is, of course, still much to be learned, the use of wetlands for wastewater treatment is no longer a novel, experimental idea, but rather an accepted technology that is beginning to mature and to diffuse throughout the U.S. and elsewhere. The focus of the chapter is on treatment of domestic sewage with wetlands, which was the first application of the technology, but many other kinds of wastewaters (urban stormwater runoff, agricultural and industrial pollution, and acid mine drainage) are now treated with wetlands.