Legionella is a bacteria that resides in the environment, usually contracted by inhalation of contaminated aerosolized water from human made water systems. Exposure can result in Pontiac fever, a self-limited flu-like illness or Legionnaires’ disease, a severe form of pneumonia. This three-part series will provide a practical overview of public health investigations of legionellosis, including assessment of the epidemiology, potential environmental sources, appropriate clinical and environmental testing, and environmental control and remediation.