Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis, is a complex syndrome of varying intensity, clinical presentation, and natural history, rather than a single uniform disease. HP is caused by sensitization to repeated inhalation of dusts containing organic antigens. These dusts can be derived from a variety of sources, such as dairy and grain products, animal dander and protein, wood bark, and water reservoir vaporizers. The most common antigens are thermophilic actinomycetes and avian proteins; the most common diseases are farmer\'s lung and bird fancier\'s lung. HP is characterized by diffuse inflammation of lung parenchyma and airways in previously sensitized patients. Based on the length and intensity of exposure and subsequent duration of illness, clinical presentations of HP are categorized as acute, subacute (intermittent), and chronic progressive.