Allogeneic transplantation of hematopoietic stem cells (HSC) is a curative treatment for hematological malignancies aiming to eradicate the malignant clone using the immunological conflict inherent to donor HSC installation in the recipient. The different possible sources of HSCs (bone marrow, blood, and cord blood) and better knowledge of HLA typing has led to the development of new transplantation techniques and modalities (transplantations after non-myeloablative conditioning, haploidentical transplantations, etc.), which should improve patient survival and extend allograft indications. HSC allografting is subject to immunological reactions stemming from the histocompatibility discrepancy between donor and recipient. For the most part, these are reactions of the graft against the host (graft-versus-host disease: GVHD) and graft rejection (host-versus-graft: HVG). This immunological conflict can also be responsible for recognizing and destroying the recipient's residual tumor cells, which carry specific tumor antigens and/or minor antigens of histocompatibility (graft-versus-leukemia effect, GVL or graft-versus-malignancy effect, GVM). The posttransplantation period can also be riddled with various complications such as veno-occlusive disease, endocrine complications, as well as complications arising from infections and secondary neoplasms because of a more or less substantial and durable immune deficiency. Acute and chronic leukemias are the major indications for HSC allogeneic transplantation, for which the results are variable and closely related to the patient status, the hematological disease, and the transplant procedure. Other hematological diseases are also indications for allogeneic transplantation but are rarer, for which allogeneic transplantation remains nevertheless the only curative treatment, despite an overly high level of toxicity. Improvement in the results of unrelated transplantations, use of peripheral HSC or cord blood cells, development of non-myeloablative conditioning regimens, and techniques of ex vivo manipulation of the graft have allowed HSC allogeneic transplantation indications to be extended. The antitumor efficacy of donor lymphocytes infusion for relapses after transplantation mirrors the GVL effect and is the first stage in a targeted cellular immunotherapy using sensitized lymphocytes or dendritic cells.