The Philadelphia chromosome-negative myeloproliferative disorders (MPDs) polycythemia vera (PV), essential thrombocytosis (ET) and primary myelofibrosis (PMF) are characterized by increased proliferation of terminally differentiated myeloid cells. Although these disorders were recognized as clonal hematopoietic stem cell disorders more than 3 decades ago, little was known about the genetic basis for these disorders until 2005 when a single recurrent mutation in the JAK2 tyrosine kinase (JAK2V617F) was identified in >90% of patients with PV and in a significant proportion of patients with ET and PMF. JAK2V617F is a constitutively active tyrosine kinase and has transforming properties in vitro and in vivo, providing validation JAK2V617F is a bona fide oncogene which contributes to MPD pathogenesis. Subsequent studies of JAK2V617F- negative MPDs have identified mutations in JAK2 exon 12 and MPL, and these mutations also result in constitutive activation of JAK2 signaling. In this review, we will discuss the genetics of PV, ET and PMF with regard to known somatic mutations, the role of these mutations in hematopoietic transformation and the therapeutic implications of these findings.