A review with 20 refs. is given on biofilms which grow at the inner walls of pipes, reservoirs, and other surfaces in contact with drinking water. A summary on the different sorts of organisms in biofilms, their population d., and an evaluation of their contamination potential in the drinking water supply system are included. Further topics include the nutrition opportunities for the biofilm growth and the microbiol. caused corrosion. The aptitude of biofilms as habitat for hygienically relevant microorganisms and their tolerance against disinfection substances are introduced.