The interstitial lung diseases are comprised of a group of pulmonary disorders characterized clinically by diffuse infiltrates on the chest radiograph and histologically by distortion of the gas exchanging portion of the lung. The physiologic correlates are restriction of lung volumes and impaired oxygenation. The term "interstitial" when applied to these diseases is actually a misnomer because it implies that the inflammatory process is limited specifically to the area between the alveolar epithelial and capillary endothelial basement membranes. The diseases currently grouped as "interstitial" also frequently involve the alveolar epithelium, alveolar space, pulmonary microvasculature, and less commonly, the respiratory bronchioles, larger airways, and even the pleura. The enormous differential diagnosis of interstitial lung disease can be made manageable by understanding that pneumoconiosis, drug-induced disease, and hypersensitivity pneumonitis account for over 80% of the responsible entities and can usually be identified from the patient's history. The nine remaining diseases/disease categories include: sarcoidosis, idiopathic pulmonary fibrosis, bronchiolitis obliterans-organizing pneumonia, histiocytosis X, chronic eosinophilic pneumonia, collagen vascular disease-associated interstitial lung disease, granulomatous vasculitis (Wegener's granulomatosis, Churg-Strauss syndrome, lymphomatoid granulomatosis), Goodpasture's syndrome, and pulmonary alveolar proteinosis. The diagnosis of a specific interstitial lung disease can be made via various means including the patient's history, specific serologies, bronchoalveolar lavage, transbronchial biopsy, and biopsy of extrathoracic tissues or open lung biopsy. A directed diagnostic approach can be formulated based on an understanding of these techniques and a thorough knowledge of the clinical presentations and specific diagnostic criteria for each of the major diseases. This monograph will serve as a guide for the clinician to use in evaluating and treating patients with interstitial lung disease. We begin by reviewing the clinical presentation, diagnostic criteria, and management of specific interstitial lung diseases excluding pulmonary infection, neoplasm, and sarcoidosis. Pneumoconiosis and drug-induced syndromes are not discussed in detail, but the agents responsible and pertinent exposures are presented in tabular form in the discussion of the general diagnostic approach.