The present invention relates to a combination of placental stem cells and stem or progenitor cells derived from a second source, wherein the combination shows improved engraftment as compared to placental stem cells or stem cells from a second source, alone. The combination is referred to as a combined stem cell population. The invention also provides in vitro and in vivo methods for identifying and producing combined stem cell populations, and models of engraftment. In accordance with the present invention, the placental stem cells may be combined with, e.g., umbilical cord blood-derived stem or progenitor cells, fetal or neonatal stem cells or progenitor cells, adult stem cells or progenitor cells, hematopoietic stem cells or progenitor cells, stem or progenitor cells derived from bone marrow, etc.