Many researchers and clinicians wonder if gene therapy remains a way to treat genetic or acquired life-threatening diseases. For the last few years, many experimental, pre-clinical, and clinical data have been published showing that it is possible to transfer with relatively high efficiency new genetic information (transgene) in many cells or tissues including both hematopoietic progenitor cells and differentiated cells. Based on experimental works, addition of the normal gene to cells with deletions, mutations, or alterations of the corresponding endogenous one has been shown to reverse the phenotype and to restore (in some case) the functional defect. In spite of very attractive preliminary results, however, suggesting the feasibility and safety of this process, therapeutically efficient gene transfer and expression in targeted cells or tissues must be proven. In this review, we will focus primarily on the attempts to use gene transfer in hematopoietic stem cells as a model for more general genetic manipulations of stem cells. Hematopoietic stem cells are included in a subset of bone marrow, cord blood, or peripheral blood cells identified by the expression of the CD34 antigen on their membrane.