The myeloproliferative neoplasm (MPN), polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF) are clonal hematopoietic stem cell diseases that share in common overproduction of one or more of the formed elements of the blood with overlapping clinical features but exhibit different natural histories and different therapeutic requirements. Therefore, accuracy of diagnosis is the cornerstone of therapy. The World Health Organization diagnostic criteria for both the classic BCR-ABL-negative MPNs (that is PV, ET, and PMF) and chronic eosinophilic leukemia/hypereosinophilic syndrome have been revised in the 2008 edition, by incorporating new information on their V617F mutation in the Janus kinase 2 (JAK2) tyrosine kinase. The JAK2 V617F point mutation makes the normal hematopoietic progenitor cells hypersensitive to thrombopoietin, erythropoietin, and myeloid progenitor cells, leading to trilinear hematopoietic myeloproliferation. JAK2 V617F is found in most patients with PV, ET, or PMF and is, therefore, useful as a clonal marker when present. However their absence does not exclude the diagnosis of an MPN. The major complications of the MPN are thrombosis, hemorrhage and extramedullary hematopoiesis with massive splenomegaly and bone marrow failure. Myelofibrosis is classically listed as a complication of the MPN. Current treatment options are low dose aspirin, phlebotomy and cytoreductive therapy with hydroxyurea, anagrelide, and interferon for PV and ET but the most effective therapy is still bone marrow transplantation for PMF for the relief of symptoms and the prevention of complications. Drugs targeting JAK2 V617F are promising. This article reviews the changes in diagnostic criteria and algorithms, and also provides treatment guidelines that are tailored to routine clinical practice.