Bronchoalveolar lavage (BAL) has become an established method for the diagnosis of infectious and interstitial lung diseases. Using a bronchoscope material from the peripheral airways and alveolar spaces can be obtained. When standard protocols are followed the procedure is generally well tolerated and in combination with clinical tests allows a reliable diagnosis in many cases. Certain clinical entities, e.g. alveolar proteinosis and Langerhans cell histiocytosis can be diagnosed by BAL alone without resorting to invasive diagnostic tests. In other diseases, BAL allows to narrow the range of the differential diagnosis. However, biopsies can often still not be avoided completely. A combination of both methods together with the clinical findings represents a powerful tool for diagnosis and research into the pathologic alterations in inflammatory and fibroising lung diseases.