Following the discovery of Legionella pneumophila as the cause of an epidemic of pneumonia at an American Legion Convention in Philadelphia, a group of related bacteria were recognized as additional human pathogens. This newly established bacteria genus, Legionella, includes the agents of Legionnaires' Disease, Pittsburgh pneumonia and several related infections. A number of researches have been performed in the past few years about these bacteria; many of these data are here summarized to give an idea of the most important characteristics of Legionella and of the diseases they cause.