Shortly after the introduction of the technique of bronchoalveolar lavage (BAL) through a flexible fiberoptic bronchoscope (1), it was apparent that the inflammatory cells retrieved by BAL were different in interstitial lung diseases versus normal volunteers (2-5). This led clinicians to begin using the results of BAL to manage patients with various interstitial lung diseases (5,6). This has been a controversial concept (7,8). In this chapter, we will discuss some of the background information that research using BAL has taught us about interstitial lung diseases, how that information has been used in the diagnosis and management of interstitial lung diseases, and the technical aspects of BAL which appear to be important to make the results understandable.