Our rivers and lakes are continuously self-purifying thanks to algal and bacterial biofilms that grow over the surface of stones and other debris. This same process has been employed for over a century to treat our municipal and industrial wastewater in specially designed fixed film reactors that maximize this microbial activity by providing ideal growth conditions and unlimited food and oxygen. Fixed film, or attached biofilm, reactors are unique in their ability to treat complex wastewaters and shock loadings; using far less energy than other wastewater treatment processes such as activated sludge, making them a sustainable treatment option. Targeted at undergraduate and postgraduate engineers and scientists, this book follows the structure of bestseller Biology of Wastewater Treatment. This volume gives an expanded and up-to-date overview of the use of fixed-film reactors in wastewater treatment with content spanning from biofilm formation, to traditional trickling filters and rotating biological contactor technology, advanced submerged systems (including MBBRs and IFAS) and their key role in the treatment of contaminated air, and finally to nitrogen removal employing new microbial pathways such as Anammox. This monograph emphasizes the biological aspects of the processes.