Wastewater collection systems, or sewers, are crucial sections of the urban water cycle where complex microbial, chemical and physicochemical processes take place. This chapter aims to give an overview of the diversity and importance of biofilms and bioreactions occurring in sewers, paying special attention to its detrimental effects. Sewer biofilms can be divided in two main classes: (1) Submerged biofilms: including activities of Sulfate Reducing Bacteria (SRB) responsible for the formation of sulfide (H2S, an odorous, toxic and corrosion-inducer compound), Methanogenic Archaea (MA) responsible for the formation of methane (CH4, an explosive and potent greenhouse gas) and the Fermentation processes that increase the two previous biofilms metabolism. (2) Unsubmerged biofilms: activities of biofilms growing on the gas phase of sewers that causes loss of concrete mass, cracking of the sewer pipes and ultimately, structural collapse. This process is known as microbially induced concrete corrosion (MICC). The structure of sewer-biofilms and mechanisms for the control of its harmful effects are described.