Small airways disease includes a spectrum of inflammatory and fibrotic pulmonary diseases centered on the small conducting airways. High-resolution computed tomography plays a key role in the detection and classification of small airways disease and, when combined with relevant clinical and pathologic findings, leads to a more accurate diagnosis. The imaging manifestations of small airways disease on high-resolution computed tomography may be direct or indirect signs of small airway involvement and include centrilobular nodules and branching nodular (tree-in-bud) opacities, or the demonstration of mosaic attenuation that is typically exaggerated on expiratory computed tomography. This article reviews the normal anatomy and histology of bronchioles and the clinical, pathologic, and imaging features of small airways diseases.