Legionella was discovered in the first half of the 20th century. The main representative of the genus is the bacterial species Legionella pneumophila. Legionella can cause a mild disease with fever but also severe to fatal pneumonia. At highest risk are individuals with an underlying disease, immunosuppressed patients or individuals exposed to other risk factors (e.g. users of addictive substances). Information on the etiology and epidemiology of legionellosis is presented. Selected risk factors are described as well as preventive measures to be taken in water supply and cooling systems. In conclusion, emphasis is placed on the prevention.