Blood collected from the umbilical cord and placenta at the birth of a child is a rich source of immature blood cell elements and has been used clinically as an alternative source of transplantable stem and progenitor cells. Studies on the proliferative and replating capacities of cord blood stem and progenitor cells have documented their extensive capacity for division and self renewal. Studies on the immune cells in cord blood have shown them to be less immunologically reactive in a number of situations. These characteristics are consistent with the experience in children receiving HLA-matched sibling cord blood cells, in which these cells have been transplantable in a large number of clinical disorders with low or absent graft-versus-host disease. Stem and progenitor cells from cord blood are efficiently transduced with new genetic material by retroviral and adeno-associated viral vectors and may be of efficacy in the future for autologous gene therapy approaches to treat disease. Efforts in banking of cryopreserved cord blood cells have been undertaken, and a number of such stored samples have been used for fully and partially HLA-matched unrelated transplantation. Efforts to better understand the cells in cord blood and their clinical utility are continuing.