This article reviews current knowledge concerning enzootic pneumonia. Enzootic pneumonia, caused by Mycoplasma hyopneumoniae and exacerbated by secondary invaders, occurs worldwide and has been known for many years. The organism, with its typical characteristics, invades the respiratory tract in several successive steps. Clinical symptoms and lesion development are the result of the pathogenic capacity of Mycoplasma hyopneumoniae and the defence reactions in the lung. The economic relevance of pneumonia is influenced to a large extent by common secondary infections which follow an initial M. hyopneumoniae infection. Different tests for the diagnosis of pneumonia in individual pigs and in groups are available. Treatment and control is not simple since enzootic pneumonia is a multi-factorial disease. Some epidemiological aspects and the most important measures for prevention of the disease are described.