Classic myeloproliferative neoplasms, including essential thrombocythemia, polycythemia vera, and primary myelofibrosis, are chronic, clonal hematopoietic stem-cell disorders. These disorders are driven by gain-of-function mutations in the genes Janus kinase 2 (JAK2), calreticulin (CALR), or the thrombopoietin receptor (MPL) that activate cytokine signaling. These mutations arise decades before clinical disease develops and confer a clonal advantage that is further shaped by comutations in epigenetic, splicing, or signaling genes. Inflammation enhances clonal dominance, favoring the development of myelofibrosis and thrombotic complications. Disease evolution may culminate in secondary acute myeloid leukemia, which has a poor prognosis.