A review, with 73 refs. Biofilms are ubiquitous in drinking water systems, either as thin and patchy colonization layer or as surface covering thick deposit. In biofilters they are used for the elimination of biol. degradable substances. However, they occur in other sites, e.g., on the walls of containers and pipes, sediments, and on suspended particles. They can rise problems by contamination of the water phase with biofilm organisms. Biofilms provide a habitat for hygienically relevant microbes in which these can persist and even multiply. Here they are protected against disinfectants, in particular if located in corrosion products, sediments or ingested by protozoa which feed on biofilm cells. Biofilms are related to the occurrence of \"black water\" and malodors. They are involved in the corrosion of metals, mineral materials, and synthetic polymers. Limiting factor for biofilm growth is usually the availability of nutrients, mainly provided either by biodegradable substances leaching from materials or from the water phase. The extent of biofilm growth and the occurrence of hygienically relevant organisms originating from biofilms is still largely unknown and requires to be investigated.