Hypersensitivity pneumonitis (HP) represents a group of immunologically mediated lung disorders provoked by recurrent exposure to various environmental agents. HP is multifaceted and may mimic almost any interstitial lung disease, some infectious diseases,and even bronchiolar disorders. In the absence of a diagnostic gold standard,diagnosis of HP requires a combination of clinical, environmental, radiologic, physiologic,and pathologic findings that represent a diagnostic challenge for clinicians and-in the chronic form-even for experienced pathologists. Therapeutic approach includes avoiding further exposure and, depending on the clinical form, the administration of a course of prednisone. New anti-inflammatory, immunoregulatory, and antifibrotic drugs are urgently needed for this and other interstitial lung diseases.