An Introduction to Constructed Wetlands highlights the part of the constructed wetland system which plays a pivotal role in the sustainable treatment of wastewater, with a major focus on possible mechanisms for the removal of polychlorinated biphenyl. The authors provide a state-of-the-art and up-to-date review of the literature on constructed wetlands, emphasising their performance in treating oily effluents. In addition, the functions of constructed wetlands as nature-based solutions are reviewed, with emphasis on their vegetation and associated microorganisms. In closing, the major findings related to the identification of the microbial community composition in the constructed wetlands bed media by the application of pyrosequencing of 16S rRNA genes.