Diseases characterized by pulmonary infiltrates and peripheral eosinophilia have been grouped based on a common clinical presentation. Early classification schemes viewed these syndromes as a continuum with significant overlap between categories. Although understanding of certain of these syndromes has increased, this classification system remains a useful framework for identification and diagnosis. At present, we cannot predict which patients with isolated lung involvement will progress to involvement of other organs. Early diagnosis and close follow-up are critical. Corticosteroids are the primary treatment in most of these diseases; cytotoxic agents also have a role. The role of the eosinophil in the disease process and the syndromes included in the differential diagnosis are reviewed. Loeffler's syndrome, eosinophilic pneumonia, the hypereosinophilic syndrome, allergic bronchopulmonary aspergillosis, Churg-Strauss syndrome, and tropical pulmonary eosinophilia are discussed in detail.