This Addendum to the second edition of the Guidelines for drinking-water quality contains reviews of a number of pathogens that occur widely in water - Aeromonas and Legionella spp., Vibrio cholerae, enteric hepatitis viruses, and protozoan parasites (Cryptosporidium, Giardia, and Cyclospora spp.). These review documents supersede, and are more comprehensive than, those previously published in Volumes 1 and 2 of the Guidelines.Unlike the analogous chemical reviews in the Guidelines, the microbiological review documents do not conclude with "safe" or "tolerable" exposure levels for the pathogens. The microbiological quality of drinking-water can vary rapidly and widely, and even brief exposures to pathogens may have serious health consequences. Analytical techniques for recognized pathogens may be time-consuming and complex, and are not always available, and knowledge of the identity of waterborne pathogens is in any case incomplete. The microbiological reviews therefore summarize current knowledge of transmission, attenuation, and removal of the individual pathogens, and of the effectiveness of measures for interrupting transmission.