The term "bronchiolitis" refers to a broad morphologic spectrum of inflammatory events that are centered on small conducting airways. Bronchiolitis may be an isolated pathologic finding, although it is often a secondary consequence of diseases affecting other parts of the conducting apparatus or pulmonary acinus. Divergent causes of bronchiolitis may have similar microscopic findings. Hence, a pathologic diagnosis is nonspecific and not clinically meaningful unless placed in the context of relevant clinical and radiographic findings. Etiologically, most cases of bronchiolitis are infectious in nature or related to smoking. Increasingly, other causes of bronchiolitis have been recognized, including collagen vascular disease, idiopathic inflammatory bowel disease, or toxins/drugs. Many cases are idiopathic. Histologically, most examples of bronchiolitis demonstrate acute inflammation, usually corresponding to viral infection. Other well-defined histologic categories of bronchiolitis include bronchiolitis obliterans, in which there is fibroblast proliferation within airspaces, and follicular bronchiolitis, corresponding to lymphoid hyperplasia with formation of germinal centers. In recent years, several new forms of bronchiolitis have been recognized, including constrictive bronchiolitis, diffuse panbronchiolitis, and airway-centered fibrosis. This article highlights the clinical and pathologic features of these more recently described entities.