Bronchoalveolar lavage (BAL) is a minimally invasive procedure used to characterize the status of the alveolar space. Standardization of the procedure and the analysis of samples taken is essential for their proper interpretation. In nonresolving or ventilator-associated pneumonia, BAL contributes to the detection of resistant pathogens and noninfectious etiologies. In immunocompromised hosts with radiological infiltrates, BAL should be performed early during work-up since outcome is significantly modified in this population group. In cases of interstitial lung disease, BAL can exclude infectious or neoplastic causes. Associated with a clinical and radiological evaluation, it provides valuables additional diagnostic information.