The diagnosis of legionellosis, especially of its severe, life-threatening form, Legionnaires' disease, is complicated, primarily because of non-typical symptoms of the infection, not always dominating atypical pneumonia, and often a very dramatic septic course of the disease with multiorgan failures. The diagnosis of the acute phase of the disease can be established by the detection of Legionella antigen in urine and by PCR/real-time PCR detection of Legionella DNA in serum and lower respiratory tract and urine samples. Cultivation on specific media remains the gold standard, but this very demanding method is rarely used. Serological testing requires paired samples and thus is relevant to the diagnosis at a later stage of infection, although it is to be noted that about 20% of patients do not produce the antibodies. Great progress has been made in typing methods (RFLP, PFGE, or PCR based and sequence based methods) and rapid identification methods (MALDI-TOF).