The application of gene transfer technology to human gene therapy has been intensively investigated during the last decade. The first human clinical trials in adenosine deaminase deficiency and metastatic melanoma recently demonstrated the feasibility and safety of using retroviral gene transduction for human gene therapy. Many problems remain to be solved for a better understanding of the functioning of genes to be transferred, a better efficiency of gene transfer and genetic correction by site-specific targeting in vivo, a better knowledge of the biological properties of target cells such as totipotent hematopoïetic stem cells. Clinical developments are expected in genetic diseases (immunodeficiency, thalassemia, hemophilia) and non genetic disorders (lymphokine gene therapy for cancer).