Legionella pneumophila was recognized as an important human pathogen after the first discovery during an investigation of a pneumonia outbreak among American Legion convention in 1976 in Philadelphia, USA. L. pneumophila is a gram-negative, mesophilic, facultative intracellular parasitic and nonspore-forming rod-shaped bacterium belonging to the gamma-subgroup of proteobacteria. L. pneumophila inhabits natural freshwater environments at low concentration. Along with the transfer from natural aquatic habitats into man-made water systems such as cooling towers, evaporative condensers, water distribution systems, whirlpool spas and hot water tanks, L. pneumophila reaches high cell density and can cause Legionnaires' disease (pneumonic legionellosis) or Pontiac fever (severe influenza-like illness). Infection occurs primarily via the inhalation of L. pneumophila-contaminated aerosols. In aquatic habitats, L. pneumophila cells are intracellular parasites of freshwater protozoa and use a similar mechanism to multiply within mammalian cells. L. pneumophila can also multiply extracellularly within biofilms and can persist within these microbial communities for years. Transmission to human primarily occurs via the inhalation of L. pneumophila containing aerosols. The bacterium enters to human phagocytic cells by coiling or conventional phagocytosis then inhibits phagosome-lysosome fusion and multiplies in the phagosome. A number of virulence factors have been described for L. pneumophila such as surface proteins, secreted factors and putative virulence factors. L. pneumophila can be identified by using cultural, serologic and various molecular techniques such as DNA sequencing and DNA-DNA hybridization. Diagnosis can be made by culture, direct fluorescent antibody staining, serological tests, urinary antigen detection or nucleic acid detection and various subtyping techniques. In order to eradicate L. pneumophila from contaminated water systems several methods are available; Thermal or chemical shock \disinfection, UV irradiation, ozone treatment, silver-copper ionization, anodic oxidation and chlorine dioxide application.