The performance of biocatalytic reactions is often hampered by product and/or substrate toxicity and relatively short reaction times, due to unstable biocatalysts. Microbial biofilms show a remarkable resistance against biocides and form physically stable communities. I nature, especially in environments characterized by harsh conditions such as heavily contaminated sites, cells grow predominantly in biofilms, which enable them to cope with physiological stress. This robustness was utilized to design a bioprocess concept based on catalytic biofilms for stable long-term transformations of toxic reactants.