The clinical importance of bone marrow transplantation has increased considerably in recent decades. More than 450 centers for bone marrow transplantation now exist worldwide, performing more than 5000 transplantations per year. The number of transplant centers and patients has increased dramatically in Turkey in the last several years. When transplant donor and recipient are different individuals, hematopoietic graft is known as allogeneic graft. Total body irradiation (TBI) remains an important component of allogeneic hematopoietic stem cell transplant, with the goal of eradicating residual malignant cells and modulating the immune system of the transplant recipient. TBI is especially advantageous in allogeneic stem cell transplantation, since its biological effects can be exerted uniformly without sparing the "sanctuary" sites, such as nervous system or testicles. In this report, the role of TBI in allogeneic stem cell transplantation is reviewed.