Myeloproliferative disorders, including chronic idiopathic myelofibrosis (CIMF), polycythemia vera (PV), essential thrombocythemia (ET), and chronic myelomonocytic leukemia (CMML), are clonal diseases of hematopoietic stem or precursor cells. They often show a protracted or chronic course; however, all have the potential of progressing to severe marrow failure, associated with myelofibrosis, or of transforming into acute leukemia. At that point, hematopoietic cell transplantation (HCT) is the only current treatment strategy with curative potential. If transplantation is being considered and a suitable donor is available, HCT should be carried out before leukemic transformation has occurred, as the success rate of HCT declines steeply in patients who have evolved to leukemia. As many as 75?80% of patients with the original diagnoses of PV or ET, about 65?70% with CIMF, and 45% of patients with CMML are surviving long term after allogeneic HCT using conventional transplant regimens, with follow-up now extending to 15 years. Results with HLA-identical related and unrelated donors are comparable. Major risk factors for the outcome after HCT are the disease stage, the presence of comorbid conditions, and patient age. The development of reduced-intensity conditioning regimens has allowed for successful HCT even for older patients and patients with comorbid conditions. Studies on disease mechanisms, including the recent characterization of an activating mutation in JAK2, may provide additional prognostic guidance and are likely to lead to the development of novel treatment strategies, which will require continuous reassessment as to the optimum timing of HCT.