Hematopoietic cell transplantation (HCT) is now a curative option for certain categories of patients with hematologic malignancies and other life-threatening illnesses. Technical and supportive care has resulted in survival rates that exceed 70% for those who survive the first 2 years after HCT. However, long-term survivors carry a high burden of morbidity, including endocrinopathies, musculoskeletal disorders, cardiopulmonary compromise, and subsequent malignancies. Understanding the etiologic pathways that lead to specific post-HCT morbidities is critical to developing targeted prevention and intervention strategies. Understanding the molecular underpinnings associated with graft-versus-host disease (GVHD), organ toxicity, relapse, opportunistic infection, and other long-term complications now recognized as health care concerns will have significant impact on translational research aimed at developing novel targeted therapies for controlling chronic GVHD, facilitating tolerance and immune reconstitution, reducing risk of relapse and secondary malignancies, minimizing chronic metabolic disorders, and improving quality of life. However, several methodological challenges exist in achieving these goals; these issues are discussed in detail in this paper. © 2011 American Society for Blood and Marrow Transplantation.