OBJECTIVE To review the outcomes of allogeneic transplantation with regimens of varying total body irradiation (TBI) doses (0-1575 cGy), with an emphasis on reduced-intensity conditioning (RIC) regimens. RESULTS RIC regimens with a broad range of TBI doses (0, 200, 400, 550, and 800 cGy) have been studied. Durable donor stem cell engraftment occurred in most patients, with a low rate of toxicity and nonrelapse mortality. Patients excluded from myeloablative regimens were able to tolerate RIC regimens. Retrospective comparisons of patients treated with RIC and myeloablative regimens showed lower nonrelapse mortality but higher relapse risk with RIC; however, overall survivals were similar though a larger fraction of RIC patients had high-risk pretransplant features. Study design weaknesses limit the ability to generalize and apply these results. RIC and myeloablative TBI-based regimens result in durable engraftment of donor stem cells, tolerable toxicity, and acceptable rates of relapse risk and overall survival. Determination of the optimal dose of TBI for allogeneic transplantation is complex and depends on several variables and may vary based on a specific permutation of these variables. CONCLUSIONS A broad range of TBI doses for RIC regimens are effective; however, all conclusions are limited by the lack of prospective, randomized trials comparing RIC and myeloablative regimens.