For patients with relapsed or refractory Hodgkin's or non-Hodgkin's lymphomas, allogeneic hematopoietic stem cell transplantation (HSCT) is a treatment option when autologous HSCT fails to achieve durable remission or is deemed inappropriate. Allogeneic HSCT can result in long-term survival even in patients with refractory lymphomas. The efficacy of allogeneic HSCT is attributed, at least in part, to an immune-mediated graft-versus-lymphoma (GVL) effect that can also be associated with significant toxicity resulting from graft-versus-host disease. However, clinical evidence of a potent GVL effect is inconsistent. Reduced-intensity conditioning before allogeneic HSCT can facilitate the use of this treatment in older patients and those at high risk. The decrease in toxicity with reduced-intensity regimens may be associated with a loss of antitumor effects. Patients with lymphoma should be selected for allogeneic HSCT on the basis of characteristics that strongly influence transplant outcomes, including histology, chemosensitivity, and donor source.