Although allogeneic stem cell transplantation can cure patients with hematologic malignancies, limiting factors such as lack of suitable donors and GVHD toxicity have led to the exploration of umbilical cord blood (UCB) as an alternative source of hematopoietic stem cells. This overview examines the advantages and disadvantages of UCB as a donor source and reviews data from preclinical and clinical studies. Ethical issues regarding UCB transplantation are also addressed.