Allogeneic hematopoietic stem cell transplantation provides many patients, with hematological and malignant diseases, hope of remission and in some cases cure. Because the toxicities of this approach are severe, its use has been limited to younger healthier patients. Nonmyeloablative and reduced intensity conditioning regimens depend more on donor cellular immune effects and less on the cytotoxic effects of the conditioning regimen to eradicate the underlying disease. This approach is based on the induction of host tolerance to donor cells followed by the administration of scheduled donor T-lymphocytes infusions. Accumulated clinical data have been encouraging, and prospective studies are underway to compare this approach to conventional myeloablative allogeneic stem cell transplantation with regard to outcome, durability of responses, effects on the immune system, and the consequences of late complications such as chronic graft-versus-host disease.