Allogeneic stem cell transplantation is an established treatment modality for a variety of hematologic malignancies. Allografting carries a high risk for complications and toxicities related to the intensive preparative regimen that is traditionally used for pretransplant myeloablation and the graft-versus-host disease–mediated by donor immune cells. Therefore, allogeneic stem cell transplantation has been used only for younger patients with a good performance status, excluding many other potential candidates because of advanced age or comorbid conditions. Nonablative, or reduced-intensity conditioning, for allogenic stem cell transplantation has been proposed as an alternative that would allow exploiting the graft-versus-tumor effect of allogeneic transplantation without the toxicity of myeloablative therapy. After more than five years of cumulative clinical experience, it is now well established that nonmyeloablative stem cell transplantation is a feasible treatment option for patients with suboptimal performance status because it routinely allows stable donor cell engraftment, is associated with low early nonrelapse mortality rates, and in chemosensitive or slow proliferating malignancies can be associated with long-term disease control. Therefore, nonmyeloablative stem cell transplantation has allowed extension of the indications of stem cell transplantation to older and more debilitated patients; however, the optimal approaches and settings for nonmyeloablative stem cell transplantation remain to be defined.