Gene transfer into hematopoietic stem cells may provide a novel treatment for a number of congenital hematologic and immunologic diseases. Retroviral vectors are currently the most effective gene transfer system, but they have significant limitations. New approaches using other vector systems, stem cells from umbilical cord blood, or cytokine-mobilized peripheral blood stem cells may increase gene transfer efficiency. The current capabilities and obstacles to successful implementation of gene therapy with hematopoietic stem cells are discussed.