The myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are clonal myeloid neoplasms characterized at the time of their initial presentation by the simultaneous presence of myelodysplastic and myeloproliferative features, which prevent them from being classified as either myelodysplastic syndrome (MDS) or myeloproliferative neoplasm (MPN). The incidence of MDS/MPN is estimated at 0.1 to 3/100,000 individuals. They are characterized by hypercellular bone marrow (BM) morphology due to proliferation in one or more of the myeloid lineages. Cytopaenias and dysplastic changes of any cell line may be seen in conjunction with elevated white blood cell (WBC) counts, thrombocytosis and organomegaly, features more commonly associated with MPN. Hepatosplenomegaly is frequently seen. The most common entities within the MDS/MPN group include chronic myelomonocytic leukaemia (CMML), atypical chronic myeloid leukaemia BCR-ABL1 negative (aCML) and juvenile myelomonocytic leukaemia (JMML), which is seen exclusively in paediatric patients. A less well-defined group of MDS/MPN-like diseases includes MDS/MPN unclassifiable (MDS/MPN-U) and a recently recognized entity of MDS/MPN with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T), previously known as refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T). It was considered a provisional entity within the group of MDS/MPN-U in the 2008 edition of the WHO, but has now been promoted to a true entry in the updated 2016 WHO edition. Since the publication of the last WHO Classification in 2008, multiple studies have examined the molecular pathogenetic features of the MDS/MPN entities (see Table 11.1). Many of these results have been incorporated into the updated 2016 WHO classification.