There are 30 or more groups of hypersensitivity pneumonitis (HP), such as farmer's lung, bird fancier's disease, humidifier lung, air-conditioner disease, and summer-type HP. Regardless of the causative agent or its environmental setting, the pathogenesis and clinical manifestations of the groups are similar. Immune-complex formation and complement activation might play a role during the early inflammatory phase of the disease. Much evidence, however, supports a more important role of T-cell-mediated delayed-type hypersensitivity reaction than humoral hyperresponsiveness in the development of the disease. High-resolution CT findings, a striking increase in the number of T cells in bronchoalveolar lavage fluids, and the presence of specific IgG and IgA antibodies to the causative antigens in the patient's serum samples are helpful in differentiating HP from other interstitial lung diseases. Management and treatment involve avoidance of antigen exposure and occasional use of corticosteroid therapy.