Hypereosinophilic syndrome (HES), chronic eosinophilic leukemia (CEL), and mast cell disease (MCD) are all considered myeloproliferative neoplasms, and diagnosis in each instance requires bone marrow examination with cytogenetic and molecular studies. HES should be distinguished from both molecularly defined and otherwise uncategorized CEL. The genes that are mutated in molecularly defined CEL include those that encode for platelet-derived growth factor receptors A and B and for fibroblast growth factor receptor 1. Diagnosis of MCD is facilitated by tryptase immunostaining and immunophenotyping to detect abnormal CD25-positive mast cells. Mutation screening for KITD816V is also advised but is not essential for the diagnosis of MCD. Asymptomatic patients with HES and no evidence of organ damage do not necessarily require immediate therapy. The same is true for patients with indolent MCD. At present, effective cytoreductive drugs for HES include corticosteroids, interferon-alpha (IFN-alpha), and hydroxyurea, imatinib for platelet-derived growth factor receptor A or B-rearranged CEL imatinib, and for MCD IFN-alpha and cladribine. In addition, a number of new drugs are currently being tested for their safety and efficacy in all 3 disorders.