Hematopoietic stem cell transplantation is one of the major advances in clinical medicine in the past 30 years or so. It is a treatment that removes malignant cells from the patient’s body through high-dose radiotherapy (i.e. pretreatment), and then transfuses pre-collected autologous or allogeneic hematopoietic stem cells back to the patient through a vein, so that the patient can rebuild normal hematopoietic and immune functions, thus achieving the purpose of radical cure. With the in-depth understanding of the characteristics of hematopoietic stem cells, advances in transplantation-related technology, and greatly improved supportive treatment, hematopoietic stem cell transplantation has gradually matured and become widely used, and is now a reliable method for curing leukemia, solid tumors, hereditary and severe immune diseases. The familiar bone marrow transplantation is only one of the common types of hematopoietic stem cell transplantation, in fact, according to its donor-recipient relationship, it can be broadly divided into autologous hematopoietic stem cell transplantation and allogeneic (allogeneic) hematopoietic stem cell transplantation (including inter-sibling and unrelated transplantation); according to the source of stem cells, it can be divided into bone marrow transplantation, peripheral blood stem cell transplantation and umbilical cord blood transplantation, etc. The main indications for allogeneic HSCT are various malignant hematologic diseases, including acute leukemia (lymphoid and non-lymphoid), chronic granulocytic leukemia; refractory hematologic diseases – severe aplastic anemia, thalassemia, etc. and genetic diseases – severe combined immunodeficiency disease. Autologous hematopoietic stem cell transplantation is mainly indicated for medium to high malignancy and relapsed refractory lymphoma, leukemia (no suitable donor), breast, ovarian and small cell lung cancer and other solid tumors that are sensitive to radiotherapy. Why should leukemia be transplanted and what is its efficacy? As we all know, chemotherapy is usually the main treatment for leukemia, and the remission rate can be as high as about 70-80%. However, despite the use of post-remission consolidation and maintenance chemotherapy, most patients still die of relapse within 1-2 years, and the long-term disease-free survival rate is only 15-35%, the main reasons for this are: 1. The dose of radiotherapy is limited by the toxicity of bone marrow treatment and cannot be increased indefinitely, while its dose size and killing power are in a 1, the dose of radiotherapy is limited by the toxicity of bone marrow treatment and cannot be increased indefinitely, while the dose size and the killing power are logarithmically related (i.e. 1:10); 2, radiotherapy has no immune anti-leukemia (tumor) effect. Hematopoietic stem cell transplantation is a new technology developed to overcome the shortage of radiotherapy alone, which not only increases the dose of radiotherapy to more than 3-5 times that of conventional chemotherapy, giving leukemia cells a fatal blow, but also rebuilds hematopoietic and immune functions through transplantation. The reconstructed immune function is different from the patient’s immune function and has anti-leukemia (tumor) effect, which further completely removes residual leukemia or tumor cells, thus leading to the eradication of leukemia. Therefore, hematopoietic stem cell transplantation is more often used for acute leukemia patients younger than 50 years of age who are in remission. Numerous studies have demonstrated that performing allogeneic bone marrow transplantation in acute leukemia improves long-term disease-free survival. In 2000, the International Bone Marrow Transplant Registry reported long-term survival rates of more than 50-60% after allogeneic bone marrow transplantation for acute leukemia. At the same time, the long-term disease-free survival rate is still better than conventional chemotherapy due to the low mortality rate associated with autologous HSCT. As for how to select patients, which transplantation method to use and when to transplant, the right decision should be made after a comprehensive evaluation. In general, for patients with chronic granulocytic leukemia, acute leukemia with relatively poor prognosis, such as those under 50 years of age (under 60 years of age for autologous transplantation), normal function of important organs, and HLA-matched donors, early transplantation is necessary to have hope for cure. For donor selection, sibling allogeneic compatibility should be preferred, followed by unrelated allogeneic donor and relative hemizygous compatibility. There are no adverse reactions or long-term effects for the donor, just like blood donation, so there is no need to be concerned.