Inflammation involving the small airways is a quite common report in pathological dissertations. However the radiologic, clinical patterns and functional impairment of adult bronchiolitis have been discussed in detail only in the last ten years. In this review a brief summary of the anatomic and histologic characteristic of small airways is reported. A pathologic classification of bronchiolitis is at first discussed. Cellular bronchiolitis, proliferative bronchiolitis with or without intraalveolar loose fibrosis (BOOP pattern), occlusive and constrictive bronchiolitis are the main patterns taken into account: peculiar subtypes (follicular bronchiolitis, diffuse panbronchiolitis, neuroendocrine cell hyperplasia with fibrous bronchiolitis) are included in the pathologic discussion. Radiologic features are reported and presented as nonspecific. HRCT Scan findings are classified with the appropriate pathologic features in: nodules and branching lines; low attenuation and mosaic perfusion; ground glass attenuation and/or alveolar consolidation. The clinical entities considered are: bronchiolitis secondary to irritant inhalation; infectious and post-infectious bronchiolitis; drug induced bronchiolitis; bronchiolitis in patients with collagen-vascular disease; diffuse panbronchiolitis; bronchiolitis in transplanted patients; neuroendocrine cell hyperplasia with fibrous bronchiolitis: cryptogenic bronchiolitis; idiopathic BOOP; respiratory bronchiolitis with interstitial lung disease (RB-ILD). Their clinical presentation, functional impairment, pathogenetic mechanisms when deemed clinically useful, BAL findings and therapeutical schemes are discussed.