There are currently 42 described species of legionellae representing 64 serogroups in the family Legionellaceae and the genus Legionella. The phenotypic characteristics of legionellae are described, including growth requirements, and biochemical characteristics. Identification of legionellae by biochemical tests, fatty acid analysis, ubiquinones, protein profiles, carbohydrate analysis, serology, monoclonal antibodies, and molecular techniques is described. The occurrence and description of Legionella-like amebic pathogens are discussed. The problems of identification to the species level are discussed, along with the need for further evaluations of additional strains from all known species using biochemical and molecular techniques.