Hypersensitivity pneumonitis is a dynamic and heterogeneous group of diseases resulting from inhalational exposure to a variety of organic and inorganic dusts. In the United States, it has predominantly been considered an occupational disease, such as in farmer's lung, bagassosis, and in the plastics and paint industry with isocyanate exposure. In Japan, however, the most common exposure to antigen has been associated with the home environment. All susceptible patient populations are at potential risk to develop this disease under appropriate antigen exposure. The clinical course can be acute, subacute, or chronic depending on the nature of the antigen and the circumstances under which exposure occurs. If unrecognized, any stage of disease has the potential to progress to endstage pulmonary fibrosis. Proper antigen identification and avoidance requires that the physician be keenly aware of its existence and the importance of the detailed occupational and home environmental history. Without a high degree of suspicion, this disease can be easily overlooked and misdiagnosed as another type of inflammatory lung disease. Although the immunologic mechanisms mediating this illness are complex and poorly understood, strict avoidance of the provocative antigen is required for optimal long-term outcome.