The chronic myeloproliferative disorders, polycythemia vera, essential thrombocythemia, and primary myelofibrosis, are clonal stem cell disorders that occur at a low frequency and mimic not only each other clinically, but also many benign and malignant hematopoietic disorders as well. The discovery that many patients with polycythemia vera, essential thrombocythemia, and primary myelofibrosis express a mutation in the Janus Kinase 2 gene (JAK2 V617F), a kinase essential for the normal development of erythrocytes, granulocytes, and platelets, provided a molecular explanation for the unregulated hematopoiesis typical of these disorders, a diagnostic test that distinguishes them from other types of myeloproliferative disorders, and an opportunity to develop targeted therapy that could potentially avoid the toxicities associated with the conventional chemotherapeutic agents currently employed in their treatment. In this review, we discuss the molecular basis of polycythemia vera, essential thrombocythemia, and primary myelofibrosis, their diagnosis and their management in the context of the JAK2 V617F mutation.