Myeloproliferative neoplasms (MPNs) are clonal bone marrow (BM) proliferations that most often manifest as increased peripheral blood (PB) counts and/or splenomegaly. Unlike myelodysplastic syndromes, the hemopoiesis in MPN is effective, leading to the increased peripheral counts; unlike the acute leukemias, there is intact maturation of all hematopoietic lineages. The main MPN entities discussed in this chapter are polycythemia vera, essential thrombocythemia, and primary myelofibrosis. The uncommon MPN entities of chronic neutrophilic leukemia and unclassifiable MPN are also discussed. An activating point mutation in the JAK2 gene underlies the pathogenesis of many MPN, including almost all cases of polycythemia vera and about half of essential thrombocythemia and primary myelofibrosis cases. Accurate diagnosis of MPN involves distinguishing these neoplasms from reactive causes of increased PB counts as well as appropriate classification of MPN into specific entities. The diagnosis of MPN requires an integration of clinical features, BM morphology, and genetic tests, in particular JAK2 mutation analysis. Therapeutic approaches, including targeted therapy as pertinent, are also discussed for each entity.