Traditionally, the microbiological quality of waters has been measured by the analysis of indicator microorganisms. The article reviews the sanitary significance of traditional indicators of faecal contamination (total coliforms, faecal coliforms and faecal streptococci) and points out their limits. For some characteristics Escherichia coli may be considered a more useful indicator then faecal coliforms and recently it has been included in all recent laws regarding fresh, marine and drinking water. A clearer taxonomic definition of faecal streptococci evidenced the difficulty into defining a specific standard methodology of enumeration and suggested the more suitable role of enterococci as indicator microorganisms. Several current laws require the detection of enterococci. The resistance of Clostridium perfringens spores may mean that they would serve as a useful indicator of the sanitary quality of sea sediments.