For most clinicians, the definition of pneumonia is the presence of an abnormal opacity on chest X-ray and symptoms of respiratory infection such as cough, mucus production, fever. The radiological signs of infection of the lower bronchial tree can be covered by other lung diseases and these signs can mimic also other lung diseases. A chest X-ray is a first chosen step in radiological imaging in patients suspected of a pulmonary infection. Only when symptoms persist and/or become worse or when the radiological imaging is unclear, a spiral CT or HRCT of the chest will be taken in consideration. The role of medical imaging in pulmonary infection is to determine the presence, localisation and extent of the infection, to detect predisposal factors, to detect complications and in the follow-up of the infection. The radiological signs are often not very typical and they have also a limited value in predicting the causal organism. However there are some radiological signs, which are very suggestive in predicting the causal organism or in predicting the way of spread of the infection.