Biofilms are ubiquitous and the consequence of extracellular polysaccharide secretions by single or multiple microbial populations on an active surface. They are one of the major contributors to biofouling and membrane failure in current membrane-based separation processes. However, biofilm-based systems have also been recognized as aiding in remediation instead of hindering the performance via biosorption and biotransformation of pollutants present in water. This chapter highlights developments in engineered microbial-biofilms-based membrane reactors to bioremediate persistent contaminants like oxyanion species and heavy metal ions. © 2019 American Chemical Society.