The anatomy and pathophysiology of obstructive airways diseases are reviewed, with emphasis on the pivotal functional roles of the respiratory and terminal bronchioles. The array of radiologic findings associated with abnormal airways, large or small, is correlated with gross pathologic changes, using surgical and autopsy cases. Examples of inflammatory changes of smaller airways are also drawn from pathologically unproven clinical material. Peripheral linear and nodular densities, which can be distinguished from vascular shadows or septal lines, are seen in association with small bronchial and bronchiolar inflammation. In many cases it is possible to distinguish airways disease patterns from those of “alveolar” or “interstitial” disease.