Human activities have altered the nitrogen (N) cycle substantially at both local, regional and global levels. As a result, the availability of reactive N in the environment has greatly increased, causing the leaching of nitrogen surface waters, groundwater and oceans and creating eutrophication of freshwater ecosystems, hypoxia in coastal waters and pollution of groundwater. Constructed wetlands (CW) can be used to remove the excess reactive N from water environment, whereas nitrate, the most common component of nitrogen in the aquatic environment, is normally removed during the denitrification. Opportunities for this approach are presented, offering examples from various parts of the world. The problems of controlling emission of nitrous oxide, which is a byproduct of denitrification and being a dangerous greenhouse gas and destroyer of ozone layer, are also discussed.