This chapter presents a state of the art summary of myelodisplastic/myeloproliferative neoplasms (MDS/MPN) to its definition, biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. MDS/MPN are clonal myeloid disorders that possibly evolve from a pluripotent lymphoid-myeloid stem cell or a more committed myeloid progenitor. MDS/MPN have an estimated incidence of up to 3 cases per 100,000. MDS/MPN-unclassifiable can be treated with imatinib mesylate at a standard dosage. Other potentially useful drugs consist of hypomethylating agents, interferon alpha, cyclosporine, thalidomide, lenalidomide, and anti-thymocyte globulin. As the most frequent subtype of MDS/MPN, chronic myelomonocytic leukemia (CMPL) is defined by persistent peripheral monocytosis and dysplasia. Chronic myelomonocytic leukemia (CMML) has an annual incidence of 1 per 100,000, a median age at diagnosis of 70 years, and a male predominance; atypical chronic myeloid leukemia is an extremely rare subtype with a median age at diagnosis in the seventh or eighth decade of life.