Cord blood has been utilized successfully for the hematopoietic reconstitution of children with lethal disorders of hematopoiesis, as an alternative to marrow derived stem cells. The majority of the transplants performed to date have utilized umbilical cord blood from HLA-identical siblings, however, much of the interest in this modality is due to its potential as a source of readily available unrelated stem cells. Cord blood offers intriguing theoretical advantages over the use of unrelated bone marrow, but additionally suffers from several limitations as well. This review is intended to highlight a number of these issues, rather than to serve as a detailed review of the clinical experience with cord blood transplantation to date.