The identification of cytokines-soluble or membrane-bound regulators of hematopoietic stem and progenitor cell survival, proliferation, and differentiation - and the definition of culture conditions that enable cell and progenitor expansion, has lead to the first clinical trials using cultured cells in addition to or in place of unmanipulated cells. The use of ex vivo expanded cells can improve several aspects of autologous and allogeneic hematopoietic cell and progenitor transplantation, such as reducing or abolishing the nadir that follows high-dose chemoradiation therapy regimens, or reducing the clinical risks associated with the use of small numbers of progenitors as in cord blood transplantation and in autologous transplantation for poor mobilizers. In addition, biological questions raised by ex vivo expansion are shared by scientists and clinicians interested in gene transfer into hematopoietic stem cells. We here review the biological problems associated with ex vivo expansion: defining efficient culture conditions, considering not only scientific and biological issues but also regulatory and commercial issues, defining appropriate surrogate endpoints that predict engraftment and superior clinical efficacy to that obtained with the use of unmanipulated grafts. We also review the results of the first clinical trials that have demonstrated the feasibilty of this approach, and have shown some of its limitations; demonstration of clinical efficacy will require more preclinical and clinical work.