Book excerpt: Environmental mycobacteria can be found in diverse environments around the world, some of which have the ability to infect animals, birds and humans and have evolved mechanisms by which they can invade and grow within host cells, the pathogenic environmental mycobacteria (PEM). Although the diseases caused by these organisms have been known for many years, it is only recently that the potential significance of PEM as a waterborne pathogen has been appreciated. This publication discusses current knowledge about the distribution of PEM in water and other parts of the environment, the routes of transmission that lead to human infection, the most significant disease symptoms that can follow infection, methods of analysis and detection, the control of PEM in drinking-water and the assessment and management of risks.