Advances in the area of tumour therapy and transplantation medicine as well as increasing numbers of immunodeficiency syndromes have resulted into growing importance of opportunistic infections. The radiological diagnosis and differential diagnosis become more difficult due to the broad spectrum of possible pathogenic organisms, pulmonary complications as part of the underlying disease and finally through possible side effects of therapy. An assignment of these changes to certain radiographic patterns seems useful. However, very few infections present with a characteristic pattern. In the majority of cases the radiography by itself cannot lead to the correct diagnosis. Here, more than in other areas, additional information and wide experience are of great importance to the radiologist.