Hypersensitivity pneumonitis is an immunologically induced lung disease caused by repeated inhalation of agents in sensitized individuals. It may present with an extensive variety of clinical abnormalities depending on the nature of the inhaled dust and the amount of exposure, the time of exposure to the antigen and the immunological response of the host. In this review, we discuss the characteristics of hypersensitivity pneumonitis based on the available information from the literature.