Patients with primary MF (PMF), post-essential thrombocythemia (ET) MF, or post-polycythemia vera (PV) MF, with intermediate-2 or high-risk disease as determined by the DIPSS, and aged ≥55 at the time of DIPSS assessment are eligible for this study. For the allogeneic HCT arm of the HLA-Matched Donor HCT Study, donors must be either 6/6 HLA-matched related donors, defined by Class I (HLA-A and -B) intermediate resolution or high resolution DNA-based typing and Class II (HLA-DRBI) at high resolution DNA-based typing (but not monozygotic twins), OR an 8/8 HLA-A, -B, -C, and -DRB1 at high resolution DNA-based typing matched unrelated donors; both peripheral blood stem cells and bone marrow grafts are allowed, and all conditioning regimen intensities and graph versus host disease (GVHD) prophylaxis regimens are allowed. For the Haploidentical Donor Study, donors must be haploidentical.