Legionella pneumophila, the causative agent of Legionnaires' disease, was first recognized in 1977 following an epidemic of acute pneumonia in Philadelphia, USA. Since then, a total of 59 Legionella species containing 80 serogroups have been characterized. Twenty-six of the Legionella species have been reported as pathogenic in humans. This review describes the microbiological characteristics of Legionella species, their habits in the environment, the source and route of infection, symptoms and diagnosis of Legionnaires' disease, and disease outbreaks in Japan.