The myeloproliferative neoplasms (MPNs) were first recognized by William Dameshek in 1951. The classic MPNs were polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF) and chronic myelogenous leukemia. They were originally grouped together based on their shared phenotype of myeloproliferation. Since then, important discoveries have been made, identifying a central role of protein tyrosine kinases in the pathogenesis of these disorders. As such, the 2008 WHO diagnostic classification for myeloproliferative neoplasms has incorporated molecular markers with histologic, clinical and laboratory information into the diagnostic algorithms for the MPNs. Important changes include (1) the change of nomenclature of myeloproliferative disorder to myeloproliferative neoplasm emphasizing the clonal nature of these disorders; (2) the classification of mast cell disease as an MPN; (3) the reorganization of the eosinophilic disorders into a molecularly defined category of PDGFRA, PDGFRB and FGFR1-associated myeloid and lymphoid neoplasms with eosinophilia and chronic eosinophilic leukemia, not otherwise specified; and (4) refinement of the diagnostic criteria for PV, ET and PMF incorporating recently described molecular markers, JAK2V617F, JAK2 exon 12 mutations and MPL mutations. This review focuses upon the important changes of the 2008 WHO diagnostic criteria for MPNs.