Hypersensitivity pneumonitis (HP) or extrinsic allergic alveolitis is a non-IgE mediated hypersensitivity disease initiated by inhalation and subsequent sensitisation to organic antigens. These diseases have been described in different occupational groups and present in acute, subacute or chronic forms based on the exposure to antigens and host response. Clinical features are dependent upon the stage of the disease and can include fever, chills, cough, dyspnoea, and weight loss. The immunopathogenesis involves both cellular immunity and antibody responses to inhaled antigens. Antibody response to the implicated antigen can be demonstrated in HP patients, but such antibodies are also detected in antigen exposed asymptomatic individuals. Bronchoalveolar lavage demonstrates lymphocytosis and preponderance of CD8+ cells. Pulmonary function studies demonstrate a restrictive pattern with diffusion defects. The diagnosis is difficult as no single test is confirmatory, hence information from clinical, radiological, physiological, and immunological evaluations may be used together for a confirmative diagnosis of hypersensitivity pneumonitis. The treatment of choice is avoidance of antigen but systemic corticosteroids may be effective in suppressing the inflammatory response. The prognosis depends on early diagnosis and effective antigen avoidance.