Bronchoalveolar lavage (BAL) has become a standard diagnostic procedure for the majority of patients with interstitial disease. The technique is safe, minimally invasive, and reveals specific information in some disorders such as pulmonary alveolar proteinosis, Langerhans cell histiocytosis, alveolar hemorrhage, malignant infiltrates, or dust exposure. Here BAL can often replace lung biopsy. The results of BAL cell differentials with a lymphocytic, a neutrophilic, an eosinophilic, or a mixed cellular pattern can be used as an adjunct to diagnosis. If a BAL finding is compatible with the suspected diagnosis in the context of an appropriate disease history and clinical and radiologic findings, this can then be sufficient for disease confirmation. Recent research focused on the pathogenesis of various types of interstitial lung disorders. In this regard, BAL findings contributed to the characterization of idiopathic pulmonary fibrosis (IPF) as a condition with a predominant T-helper-2 (Th2) cytokine profile, whereas BAL findings in sarcoidosis and hypersensitivity pneumonitis are characterized by a Th1-dominant profile. The clinical value of BAL to assess the activity of disease processes and to provide prognostic information is still under debate. The routine performance of serial BAL is not recommended at present.