The chronic myeloproliferative disorders (CMPD) are a group of clinically related diseases characterized by clonal hematopoiesis with increased proliferation of one or more myeloid cell lineages. The identification of JAK2 mutations (JAK2V617F and JAK2 exon 12) in patients with CMPD is of great significance in the understanding of the molecular mechanisms underlined the pathogenesis of the disease contributing also to clinical management of patients. However, the precise pathogenetic contribution of JAK2 mutation is far from being fully elucidated and it is currently under intense investigation. Testing of JAK2 mutations has made the diagnosis of CMPD more precise than ever before, while genotype-phenotype associations have been identified. Furthermore, the discovery of JAK2 mutations facilitated the development of new targeted therapies and clinical trials are currently ongoing.