Myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are clonal hematopoietic neoplasms that display features of both myeloproliferative neoplasms (MPN) and myelodysplastic syndromes (MDS). They typically display some degree of effective hematopoiesis, manifested by an increase in one or more peripheral counts and/or organomegaly due to extramedullary hematopoiesis. However, they also exhibit aspects of ineffective hematopoiesis with one or more cytopenias, morphologic dysplasia, and/or abnormal effector cell function. Although MDS/MPN entities have in common this combined discase phenotype, within each disease entity there is often a wide spectrum of clinical presentations, in some cases resembling “pure” MDS and in others “pure” MPN entities. Myeloblasts may be increased in MDS/MPN cases, and in some of these entities blast count defines prognostic groups as with MDS; however, the bone marrow (BM) and peripheral blood (PB) blast count is always less than 20%.