Particular attention is given to lower respiratory tract infections because of their frequency and potential severity. These infections represent a major cause of death worldwide and pneumonia remains the first cause of death from infectious origin in France. Three nosological entities are usually described according to the anatomic localization of the infectious process: acute bronchitis, pneumonia, and bronchopneumonia (if the infection involves the bronchial tree and the lung parenchyma). If bronchial infection occurs within the context of a chronic respiratory tract disease, it is called acute decompensation of chronic lung disease (usually chronic obstructive pulmonary disease). The major diagnostic difficulties are to be able to confirm alveolar implication in the infectious process and to determine the pathogenic agent(s) responsible for the clinical pattern. This information is essential for subsequent care depends. Apart from clinical examination, essential in this context, only chest-X-rays can be of any help to confirm alveolar involvement in the disease process. On the contrary, the interest of systematic microbiological confirmation and its value according to various techniques and swab conditions may be questioned for the clinical diagnosis. Microbiological confirmation does not seem essential in every case. The feasibility and relevance of microbiological techniques must be determined before any decision is taken on documentation. The microbiological documentation value depends on its yield and sensitivity profile in identifying the pathogen.