Allogeneic hematopoietic stem cell transplantation is a curative option in different hematologic malignancies. This benefit is traditionally based on the immune anti-tumour effect mediated by the allogeneic immune effectors derived from the graft, usually called "graft versus leukemia". Several categories of donors and sources of stem cells are currently used. In addition, different types of preparative regimens are available, and can be distinguished based on their myeloablative and immunosuppressive properties. Despite significant progress in terms of short term toxicity and morbidity, and despite effective prophylactic strategies, graft versus host disease remains a major complication, with its corollary of prolonged immunosuppression and opportunistic infections. However, allogeneic stem cell transplantation is rapidly expanding because the immunological anti-tumour effect has been demonstrated both in myeloid and lymphoid malignancies with a relatively acceptable risk of toxicity.