The transplantation of hematopoietic stem cells (HSCs) obtained from bone marrow (BM), mobilized peripheral blood (MPB) and umbilical cord blood (CB) has become an established therapy to regenerate the immuno-hematopoietic system in cancer patients following ablative chemo/radiotherapy and in patients with non-malignant hematological disorders. An important question in this field is how to measure the number of hematopoietic stem and progenitor cells (HSPCs) in a cellular product prior to its infusion. This information can be used to assist in graft selection and, ideally, to prospectively predict the likelihood of successful engraftment.