Transplantation of hematopoietic precursor cells is an established therapy today in the treatment of hematological malignancies. Cells from different sources [bone marrow, peripheral blood, cord blood] and from different donor types [autologous, syngeneic or allogeneic] are used for transplantation. The aim of autologous transplantation is to apply intensive high-dose chemo-radiotherapy and to shorten the duration of aplasia. Allogeneic cells, in addition, are free of potentially contaminating precursor cells and provide a graft-versus-leukemia effect. For all patients, transplantation should be considered at diagnosis as an integral part of treatment strategy and, depending on risk factors, be performed early in the course of disease. Preferred time for patients with high-risk acute leukemias is first complete remission, second complete remission for standard or low-risk acute leukemias. For chronic myeloid leukemia, allogeneic transplantation should be performed within one year of diagnosis, preferably still in first chronic phase. Autologous transplantation can be considered in a protocol setting. For patients with myelodysplastic syndromes of the FAB subtype refractory anemia or refractory anemia with sideroblasts, allogeneic transplantation is the treatment of choice as initial therapy. For patients with refractory anemia and excess of blasts with or without transformation, remission induction should be attempted before transplantation. Autologous transplantation is the preferred treatment strategy for patients with Hodgkin's and non-Hodgkin's lymphoma, for high-risk patients in first complete remission, for other patients in chemotherapy-sensitive first relapse. For patients with myeloma, transplantation should be considered after first line therapy. Age is the main individual patient's risk factor, transplant-related mortality immediately increases in parallel to increasing age. Autologous transplants are limited to patients below 60 to 65 years, allogeneic HLA-identical sibling transplants to patients below 50 to 55 years, and unrelated transplants to patients below 40 to 45 years. Prerequisites for transplant are availability of a donor, access to a transplant bed, informed consent of patient and donor, as well as financial guarantee. Indications for the different hematological malignancies and the major risk factors are discussed.