Hematopoietic stem cell transplantation (HSCT) is a critical treatment for various hematological malignancies and other conditions. This review discusses the biology of hematopoietic stem cells (HSCs), clinical efficacy of HSCT, transplantation procedures, and potential complications. HSCs are undifferentiated cells capable of self-renewal and differentiation into all mature blood lineages. They are collected from bone marrow (BM), peripheral blood, or umbilical cord blood (UCB) and used for autologous or allogeneic HSCT. Autologous HSCT uses the patient's own cells, while allogeneic HSCT uses HLA-matched donor cells. Survival after allogeneic HSCT depends on donor-recipient matching, graft-versus-host response, and graft-versus-leukemia effect. HSCT is used for treating lymphoma, leukemia, immune-deficiency illnesses, congenital metabolic defects, hemoglobinopathies, and myelodysplastic and myeloproliferative syndromes. The process involves intensive myeloablative chemoradiotherapy followed by stem cell "rescue." HSCs home to the recipient's hematopoietic microenvironment and engraft in the BM niches. The homing process involves sequential activation of adhesion molecules and is mediated by chemokine SDF-1 and vascular ligands. HSCT allows higher doses of chemotherapy, which would otherwise be fatal. Autologous or allogeneic HSCs are used as a "rescue" after myelosuppression. Allogeneic HSCT eradicates malignant cells and may confer a graft-versus-tumor (GvT) effect. HSCT is also used for congenital or acquired BM failure, immunodeficiency states, and autoimmunity. HSCs can act as "therapeutic vehicles" to replace defective enzymes or introduce genes that mediate antitumor activity. HSCT has significant complications, including infections, graft-versus-host disease (GvHD), and late complications. Infections are a major cause of morbidity and mortality after HSCT. GvHD can be classified into acute and chronic, with acute occurring within 100 days and chronic after 100 days. The severity of GvHD varies, and treatment includes corticosteroids, calcineurin inhibitors, and other immunosuppressive drugs. Chronic GvHD can lead to late morbidity and mortality, with treatment options including CsA and prednisolone. HSCT has improved survival rates, with the best 5-year survival rates for HLA-matched sibling transplants. However, long-term survivors may experience functional impairments, immune deficiencies, and other late effects. Quality of life (QoL) is affected by factors such as age, disease stage, and GvHD. HSCT is a lifelongHematopoietic stem cell transplantation (HSCT) is a critical treatment for various hematological malignancies and other conditions. This review discusses the biology of hematopoietic stem cells (HSCs), clinical efficacy of HSCT, transplantation procedures, and potential complications. HSCs are undifferentiated cells capable of self-renewal and differentiation into all mature blood lineages. They are collected from bone marrow (BM), peripheral blood, or umbilical cord blood (UCB) and used for autologous or allogeneic HSCT. Autologous HSCT uses the patient's own cells, while allogeneic HSCT uses HLA-matched donor cells. Survival after allogeneic HSCT depends on donor-recipient matching, graft-versus-host response, and graft-versus-leukemia effect. HSCT is used for treating lymphoma, leukemia, immune-deficiency illnesses, congenital metabolic defects, hemoglobinopathies, and myelodysplastic and myeloproliferative syndromes. The process involves intensive myeloablative chemoradiotherapy followed by stem cell "rescue." HSCs home to the recipient's hematopoietic microenvironment and engraft in the BM niches. The homing process involves sequential activation of adhesion molecules and is mediated by chemokine SDF-1 and vascular ligands. HSCT allows higher doses of chemotherapy, which would otherwise be fatal. Autologous or allogeneic HSCs are used as a "rescue" after myelosuppression. Allogeneic HSCT eradicates malignant cells and may confer a graft-versus-tumor (GvT) effect. HSCT is also used for congenital or acquired BM failure, immunodeficiency states, and autoimmunity. HSCs can act as "therapeutic vehicles" to replace defective enzymes or introduce genes that mediate antitumor activity. HSCT has significant complications, including infections, graft-versus-host disease (GvHD), and late complications. Infections are a major cause of morbidity and mortality after HSCT. GvHD can be classified into acute and chronic, with acute occurring within 100 days and chronic after 100 days. The severity of GvHD varies, and treatment includes corticosteroids, calcineurin inhibitors, and other immunosuppressive drugs. Chronic GvHD can lead to late morbidity and mortality, with treatment options including CsA and prednisolone. HSCT has improved survival rates, with the best 5-year survival rates for HLA-matched sibling transplants. However, long-term survivors may experience functional impairments, immune deficiencies, and other late effects. Quality of life (QoL) is affected by factors such as age, disease stage, and GvHD. HSCT is a lifelong