Indicator organisms used in the analysis of waters to show the potential presence of faecal material, and therefore pathogenic species, have historically included coliforms, faecal coliforms, Escherichia coli, enterococci, Clostridium perfringens and other sulfite-reducing clostridia; these are the key microorganisms in the assessment of the quality of drinking, surface and recreational waters. Additional tests for aerobic bacteria, “heterotrophic plate counts” in drinking water, Pseudomonas aeruginosa and Staphylococcus aureus, and the pathogens Salmonella spp., Campylobacter spp. and E. coli O157:H7 may also be of importance in determining water quality for drinking, recreational uses and in health care and food production settings.