A review with 62 refs. In water systems, biofilms are sources of contamination for the water phase and support rapid microbial regrowth. Biofilms cover surfaces. In membrane processes, this leads to an increase of the hydraulic membrane resistance. Biofilms are viscoelastic and display a more or less rough surface. Thus they consume kinetic energy and cause an increase pressure drop. In porous filter media, membrane systems, heat exchangers, water pipelines and on ship bottoms, the energy demand is increased while the performance is decreased. In heat exchangers, biofilms represent a gel layer between medium and surface. Then diffusive heat transport prevails over convective heat transport, which decreases the efficacy of the heat transfer process. The loss of performance and product quality caused by cleaning efforts due to biofouling imparts high tech. and financial damage. This is increased indirectly by countermeasures such as the application of biocides because these may promote corrosion processes and contaminate the environment.