Retroviral vectors have become standard tools for gene transfer in tissue culture and in animals. The availability of retroviral vectors and packaging cell lines brings this technology within the reach of any laboratory with standard cell culture and recombinant DNA expertise. There is preliminary evidence that retroviral vectors can be used for correction of human disease, although unequivocal proof of clinical efficacy is not yet available. In this regard, further vector development is still needed, especially of vectors suitable for use in humans that would allow the efficient transduction of nondividing cells. Vector improvements, and a better knowledge of stem cell biology, factors that control gene expression, and the response of that immune system to transferred genes, are of considerable importance if this innovative approach is to be generally used for the treatment of disease.