Human leukocyte antigen-matched sibling donor umbilical cord blood transplantation, or 0-2 human leukocyte antigen mismatched unrelated donor umbilical cord blood, is now considered an acceptable alternative to the use of bone marrow as a source for hematopoietic stem cells for pediatric hematopoietic stem cell transplantation, and is being investigated in adults. Major advantages of umbilical cord blood include the speed of availability compared with unrelated donor bone marrow, and tolerance of 1-2 human leukocyte antigen mismatch, which offers the opportunity to extend the donor pool. Umbilical cord blood transplantation is associated with durable engraftment and low incidence of severe graft-versus-host disease, even in the 1-2 human leukocyte antigen mismatched setting. Clinical experience has established the importance of graft cell dose in determining engraftment and survival in unrelated donor umbilical cord blood transplantation. More recently, the influence of human leukocyte antigen on outcome has become apparent. This review outlines the state of the art of umbilical cord blood transplantation, with emphasis on practical considerations in umbilical cord blood selection, and describes current research directions for this hematopoietic stem cell source.