Legionella are known as aquatic, gram-negative, non-spore-producing bacteria that may be transported into ambient air and dispersed by humid aerosols. The diseases that can be caused by Legionella include Pontiac fever and Legionella pneumonia (Legionnaires? disease). The standard summarises the technical knowledge on properties, occurrence and environmental health relevance of Legionella as well as information on sources, measurement methods and existing regulations. It presents possibilities and limitations of environmental health assessment and gives specific recommendations for preventive measures.