Animal xenograft models of gene therapy have become increasingly popular to study the effects of various transduction strategies on human hematopoietic stem cells (HSC). Xenograft models provide an in vivo setting in which to monitor the duration and effects of vector expression in the progeny of the transduced stem and progenitor cells. Also, the ability of HSC to home to the bone marrow and differentiate into multilineage progeny following ex vivo manipulation can only be tested in a transplantation system. The current review will discuss the murine xenograft models that have been used recently to determine optimized methods for gene transfer into normal human hematopoietic stem cells.