A wide range of pathogenic micro‐organisms may be found in wastewater and effluents associated with animals. Bacterial indicators may not always be adequate to demonstrate their presence in raw and treated waters. Some pathogens have only recently been recognized as causing problems in water, either by their resistance to disinfection or their ability to grow in aquatic systems. Much time and effort is being spent on improving existing methods and developing new methods which will improve (a) our ability to detect pathogens, (b) our understanding of their distribution in the environment, and (c) the efficacy of water treatment in removing them.