The small airways of the lung consist of the terminal bronchioles, respiratory bronchioles, and alveolar ducts. A recently introduced pathologic classification system divides bronchiolitis into proliferative and constrictive types. The histologic classification of small-airways disease into proliferative and constrictive bronchiolitis frequently correlates with the radiographic appearances. Proliferative bronchiolitis is characterized by air-space opacification, whereas constrictive bronchiolitis tends to be associated with lobular areas of decreased attenuation and airway dilatation. The purpose of this essay is to illustrate the radiographic and CT features of these two varieties of bronchiolitis.