The discovery of the JAK2V617F mutation triggered an unexpected flowering of basic and clinical studies in the field of myeloproliferative neoplasms (MPNs), resulting after just a few years in an exceptional amount of new information. One important consequence of those new findings was the modification of the World Health Organization classification and diagnostic algorithms for these diseases, which is still based on the original concept developed by William Dameshek in 1951 and keeps distinct entities under the umbrella of classical Philadelphia-negative MPNs. These MPNs are essential thrombocythemia, polycythemia vera, and primary myelofibrosis. Could a new molecular classification be a better tool to manage MPN patients? Several studies have shown that essential thrombocythemia and primary myelofibrosis can be divided into distinct subtypes based on the presence of the JAK2V617F mutation. Can we now define JAK2-positive diseases to depict a distinct entity from JAK2-negative MPNs? This chapter reviews the significance of JAK2 mutation positivity in the diagnosis, prognosis, and therapy of MPNs.