A review with 65 refs. Countermeasures against biofouling include detection, sanitation, and prevention of biofouling. The detection refers to surfaces. Cell counts in water samples do not reveal the exact location nor the extent of biofilm growth. Biocides display only limited value in terms of removal of biofouling layers. First, biofilm organisms are protected against biocides and tolerate 10 to 1000-fold higher concns. Second, water systems usually cannot be kept sterile. Thus, dead biofilms provide nutrients and suitable surfaces for further growth of cells imported with the raw water. Cleaning of a system is an integral part of sanitation and even more important than disinfection. It has to be based on a strategy. Efficiency control is mandatory and has to occur on representative surfaces. The prevention of biofouling is frequently achieved by a continuous dosage of biocides. This is, however, only possible with suitable raw waters, and many failures are reported. Chlorine is still the biocide most frequently used. Reasons of efficacy and environmental protection give rise to other strategies. \"Good housekeeping\" is recommended as a genera countermeasure. It consists of frequent cleaning, efficiency control, biofilm monitoring, limitation of nutrients, maintenance of high shear forces, and a cleaning-friendly design. Biofouling is considered as a biofilm rector in the wrong place. If large colonization areas are offered ahead of a system to be protected, they will act as a biofilm reactor and prevent biofilm growth above a \"threshold of interference\" in subsequent areas. Thus, biocide-free antifouling strategies are possible.