Over the past decade, considerable advances have been made in the field of allogeneic hematopoietic stem cell transplantation. Recognition that transplanted donor immune cells can cure patients with leukemia has led to the development of nonmyeloablative or “low-intensity” conditioning regimens, which have expanded the application of allogeneic transplantation to a growing number of hematological malignancies. The improved safety and preliminary success of this transplant approach have justified applying allogeneic immunotherapy to patients with treatment-refractory solid tumors.