Main etiologic agents of AIDS-related infections pneumonia and diagnostic tools useful to identify microorganism are discussed. Bronchoalveolar lavage (BAL) remains the most important performed technique to collect pulmonary secretions, but it needs to be completed by colorimetric, immunological (monoclonal antibodies), serological (ELISA vs. mycobacterial antigens), molecular (DNA probes) techniques. These tools are very useful to identify pathogen agents.