The study of outbreaks of Legionella pneumophila has been essential in understanding the organism, the disease, and its pathogenesis. Early epidemics defined the clinical spectrum: Pontiac fever is an acute, self-limited, febrile illness with an attack rate of 95% to 100% and an incubation period of 36 hours. In contrast, legionnaires' disease is a life-threatening bronchopneumonia with an attack rate of 2% to 7% and an incubation period of two to ten days. Three times as many males as females are affected with legionnaires' disease, and age, cigarette smoking, and chronic medical disease (particularly immunosuppression) appear to be separate risk factors. Furthermore, L pneumophila is responsible for approximately 1% to 3% of community-acquired pneumonias, 13% of those acquired in the hospital and as many as 26% of atypical pneumonias. Diverse environmental reservoirs have been identified, including cooling systems, potable or domestic water systems, respiratory therapy devices, industrial coolants, and whirlpool spas. Hot water temperature, stagnant water, sediment, and the presence of other microorganisms are important factors in the amplification of the Legionellaceae. Although airborne transmission has been widely suggested, aspiration may be an important mode in certain patients. Regional and national surveillance may identify common sources and allow the introduction of early control measures. The latter have included primarily pulse and continuous hyperchlorination and super-heating hot water systems to 50 to 60 degrees C. Experimental data suggest that ozone and UV light may be useful in the future. Additionally, cooling towers and evaporative condensers have been decontaminated and maintained with a variety of biocides. The prevention of outbreaks requires thoughtful planning, redesign, and good engineering practices.