This chapter discusses various eosinophilic myeloproliferative disorders. It provides a systematic approach to the differential diagnosis of eosinophilia and considers the role of bone marrow biopsy in diagnosis of eosinophilia. Idiopathic hypereosinophilic syndrome (HES) requires a diagnosis of exclusion. Although no obvious causes of reactive eosinophilia have emerged, a work-up for primary (clonal) eosinophilia has not yet been undertaken. The current World Health Organization classification of myeloid neoplasms provides guidance for approaching the evaluation of primary eosinophilias. Laboratory evaluation of primary eosinophilia should begin with screening of the peripheral blood for the FIP1L1–PDGFRA gene fusion by reverse transcription polymerase chain reaction (RT-PCR) or interphase or metaphase fluorescent in situ hybridization (FISH). Among treatment options, imatinib is first-line therapy in patients with FIP1L1– PDGFRA-positive disease and the rare patients with alternate PDGFRA fusions or rearranged PDGFRB; and corticosteroids (e.g., prednisone 1 mg/kg) are recommended as first-line treatment for HES.