The bacterial origin of waterborne diseases was discovered at the turn of this century. Introduction of slow sandfiltration, chlorination, and bacteriological control dates back to the same period. Although greater concern is given to-day to chemical pollutants or to viruses, bacteria are still a menace to countries with advanced water treatment. Within the last decade outbreaks were reported in Europe and the US due to Salmonella types, Shigella, E. coli, and to Vibrio cholerae, generally due to deficiencies in treatment, and often caused by smaller private works. A wider spectrum of bacteria must be taken into consideration to-day, Enterobacteriaceae are still the most important including Yersinia, E. coli, Klebsiella, and Enterobacter. The opportunistic microorganisms (pseudomonads, Acinetobacter, Campylobacter, Aeromonas, Flavobacterium) can also be a danger.