Although eosinophils play an important role in host defense mechanisms, they also are involved in many inflammatory conditions. Many pulmonary inflammatory disorders are characterized by tissue infiltration with eosinophils, with or without peripheral blood eosinophilia. Treatment often includes immunosuppression with corticosteroids, but precise diagnosis is necessary for treatment planning and prognosis. In this article, the biology of the eosinophil and several of the pulmonary eosinophilic syndromes is reviewed. In addition to the classic pulmonary eosinophilic syndromes (e.g., Churg-Strauss syndrome, allergic bronchopulmonary aspergillosis, chronic eosinophilic pneumonia), two newly described syndromes, eosinophilia-myalgia syndrome and idiopathic acute eosinophilic pneumonia, are discussed. Emphasis is placed on the clincial presentation, differential diagnosis, histologic features, and treatment strategies.