Bronchiolar disorders present a classification problem because they include a wide spectrum of diseases that often have nonspecific pathologic, clinical, and radiologic manifestations. Histopathologically, the term bronchiolitis refers to inflammation, narrowing, or obliteration of bronchioles. The inflammation may be acute (e.g., bronchiolitis with neutrophils) or chronic (bronchiolitis with only chronic inflammatory cells). Acute bronchiolitis typically is infectious, but chronic bronchiolitis may be infectious or have a variety of completely different etiologies including cigarette smoking and mineral dust exposures. We also discuss constrictive bronchiolitis (also called bronchiolitis obliterans or obliterative bronchiolitis) and fibrosing bronchiolitis, patterns that are more specific in terms of pathologic findings.