The small airways constitute a significant compartment of the lung, and diseases therein are not uncommon, occurring in a wide variety of diseases, either as a primary bronchiolar disorder or as a component of local or systemic disease affecting multiple anatomical compartments. Unlike the idiopathic interstitial pneumonias, there is as yet no consensus classification system for bronchiolar disorders, and a multidisciplinary approach to their classification is discussed. Whereas some conditions show relatively characteristic histological features, others show non-specific or subtle changes that rely on a combination of clinical, radiological and pathological information for accurate final diagnosis. Furthermore, certain diseases of the small airways have been better characterized recently, and several provisional new entities have also been described, some of these being reviewed here.