During the last two decades the multiple functions and values of wetlands have been recognized not only by the scientists and managers working with wetlands, but also by the public. The ability of wetlands to transform and store organic matter has been exploited in constructed wetlands. This paper summarizes the state-of-the-art of the uses of constructed wetlands in water pollution control by reviewing the basics of the technology, the historical development, and the performance expectations with focus on the use of free water surface and subsurface flow constructed wetlands for municipal wastewater treatment. Performance data from a total of 104 subsurface flow systems and 70 free water surface flow systems are reviewed. The present state of knowledge is sufficient to apply constructed wetlands as a tool for improving water quality. The potential applications range from secondary treatment of municipal and various types of industrial wastewaters to polishing of tertiary treated waters and diffuse pollution. In many situations constructed wetlands is the only appropriate technology available. The treatment capacity of subsurface flow systems can be improved by selecting vertical flow systems with intermittent loading, by proper media selection, and by recycling of the wastewater. Further research is needed to help define and optimize engineering design criteria and the long-term performance capabilities and operational problems.