In 2005, an activating mutation in the Janus kinase 2 (JAK2) was identified in a significant proportion of patients with myeloproliferative neoplasms, mainly polycythemia vera, essential thrombocythemia and primary myelofibrosis. Many types of mutations in the JAK-STAT pathway have been identified, the majority are related to JAK2. Currently JAK2 mutations are important in the area of diagnosis of myeloid neoplasms, but its role beyond the confirmation of clonality is growing and widening our knowledge about these disorders. In addition to that, clinical trials to target JAK2-STAT pathway will widen our knowledge and hopefully will offer more therapeutic options. In this review, we will discuss the role of JAK2 abnormalities in the pathogenesis, diagnosis, classification, severity and management of hematologic neoplasms.