Hematopoietic stem cells reside in the body’s bone marrow and are responsible for the formation of the different cells of the blood. Hematopoietic Stem Cell Transplantation (HSCT), sometimes also referred to as bone marrow transplantation, is a procedure in which hematopoietic stem cells are infused to restore bone marrow function in patients whose own bone marrow has stopped working or been destroyed, commonly because of administration of bone-marrow-toxic doses of cytotoxic drugs with or without whole body radiation therapy for treatment of a malignancy. The new stem cells can come from different sources, including the patient (autologous), a relative (allogeneic related) or an unrelated donor (allogeneic URD). HSCT is commonly performed in connection with treatment of hematological conditions and malignancies, but sometimes also for certain solid tumors, particularly in children. There are approximately 1 500 HSCTs performed in the Nordic countries each year.