This observational study will compare outcomes of a prospectively-enrolled cohort of Hematopoietic Stem Cell Transplant (HCT) recipients with outcomes of a cohort of age-matched historical non-HCT controls. Patients undergoing alloHCT will receive HCT in a US transplant center and be reported to the Center for International Blood and Marrow Transplant Research (CIBMTR) using well-established CIBMTR report forms and data collection procedures as well as a study-specific supplemental form. Data on the historical non-HCT controls will be collected at 14 US academic centers. These centers will provide data on all consecutive patients with PMF, post-ET MF, or post-PV MF referred to their institutions between 2000 and 2012.