Hematopoietic stem cells are the cells primarily responsible for short term and long term hematological reconstitution when a graft is infused into a myeloablated host. The number and quality of hematopoietic stem cells within a graft are the major determinants of the time to and durability of hematological reconstitution of a transplant recipient. Ex vivo hematopoietic stem cell expansion is, therefore, a critical component of several potentially important clinical strategies including gene therapy, tumor purging and graft engineering. Recent clinical trials using a variety of ex vivo hematopoietic stem cells expansion systems have, to date, met with limited success. Recognition of the consequences of hematopoietic stem cell self-replication will assist in the development of new approaches to hematopoietic stem cell expansion. The use of suitable in vivo models to assay the marrow repopulating potential of expanded hematopoietic stem cell products is a vital step prior to entry again into clinical trials.