The interstitial lung diseases are a heterogeneous group of chronic, non-infectious, non-malignant dis-orders that affect primarily the alveolarstructures. 1-5 The spectrum of disease included under this heading is enormous-at least 130 different inter-stitial lung diseases have been described (see references 1, 5 for the current list). Most of these diseases, however, are relatively rare; the common interstitial disorders are those resulting from inhalation of inorganic or organic dusts, sarcoidosis, idiopathic pulmonary fibrosis, and the interstitial lung diseases associated with the collagen-vascular disorders.'While diagnosis is not usually a major problem, the management of patients with interstitial lung disease presents a difficult challenge. These diseases are generally progressive, but in a baffling, often intermittent, stop-start fashion. More importantly, it is now apparent that conventional, clinical, radio-logical, and physiological assessments bear little relationship to staging the activity of these disorders, thus frustrating the clinician's attempt to make rational therapeutic decisions. 1 5 6 All this is changing, however. Through the work of many laboratories throughout the world, it is now recognised that, independent of the type of disease or specific aetiology, the earliest manifestation of interstitial disease is an" alveolitis"-the accumulation of inflammatory and immune effector cells within the alveolar structures. 1 3-5 7-10 The importance of the alveolitis is simply stated: while it is clear that the interstitial lung diseases are characterisedby a variety of derangements to the cells and connective tissue matrix that comprise the alveolar structures, it is now recognised that it is the alveolitis that causes these derangements. As we will see, the concept of" alveolitis" has caused a revolution in think-ing that has completely changed the clinical approach to staging and treating patients with interstitial lung disease.