Biofilms are important in water and wastewater treatment systems. Biofilms are aggregates of microorganisms suspended in a matrix of extracellular polymeric substances attached to a surface. Their resistance to toxic shocks and retention of slow-growing microorganisms make biofilms advantageous wastewater treatment strategies, achieving efficient removal of a variety of contaminants. Conversely, biofilm mass transfer resistance and predation protection allow them to persist in water distribution systems and other treatment infrastructure where they corrode pipes, reduce pressure head, and allow pathogen persistence. This chapter outlines fundamental biofilm processes, highlights the benefits of biofilm treatment processes, and describes strategies for biofilm control.