Reported in 1989, studies by Broxmeyer, Gluckman, and colleagues demonstrated that umbilical cord blood (UCB) is a rich source of hematopoietic stem/progenitor cells (HSPC) and that UCB could be used in clinical settings for hematopoietic cell transplantation. Since then, a great interest has been generated on the biological characterization of these cells. Over the last nine years, several groups have focused on the study of UCB HSPC, addressing different aspects, such as the frequency of these cells in UCB, the identification of different HSPC subsets based on their immunophenotype, their ability to respond to hematopoietic cytokines, the factors that control their proliferation and expansion potentials, and their capacity to reconstitute hematopoiesis in animal models. Most of these studies have shown that significant functional differences exist between HSPC from UCB and adult bone marrow (i.e., the former possess higher proliferation and expansion potential than the latter). It is also noteworthy that genetic manipulation of UCB HSPC has been achieved by several groups and that genetically modified UCB cells have already been used in the clinic. In spite of the significant advances in the characterization of these cells, we are still in the process of trying to fully understand their biology, both at the cellular and the molecular levels. In the present article, we describe and discuss what is currently known about the biology of UCB HSPC.