Wetlands are often viewed as providing a cost-effective and natural alternative treatment strategy for ameliorating pollutants during the transit of wastewater from terrestrial to aquatic systems. Natural wetlands have been used as convenient wastewater discharge sites for at least a century while artificially constructed wetlands are becoming increasingly important, as these can be bioengineered and carefully managed to cope with site-specific waste problems. It is often assumed that, as in conventional treatment systems, physical and microbial processes drive the ameliorative mechanism. However, plants are known to improve the treatment efficiency of many pollutants in wetland systems. The precise role that the plants play in these systems remains controversial, and the following discussion will highlight some of the main concepts.