—Hemophilia is an X-linked bleeding diathesis caused by a deficiency of either factor VIII or factor IX. Present treatment for hemophilia involves intravenous infusion of either recombinant or plasma-derived clotting factor concentrates. Problems with this treatment method, including the expense, need for intravenous access, and risks of blood-borne disease transmission, have fueled an interest in developing a gene-transfer approach to treatment. On the basis of experience with protein concentrate therapy, it seems likely that even modest elevations in circulating levels of factor VIII or factor IX can prevent most of the mortality and much of the morbidity associated with the disease. Hemophilia has a number of advantages as a model system for working out strategies for gene transfer as an approach to the treatment of genetic diseases; these include wide latitude in choice of target tissue, a wide therapeutic window for levels of circulating factor, ease of determining therapeutic endpoints, and existence of excellent animal models of the disease. Preclinical studies over the last decade have recently culminated in the initiation of clinical trials of gene transfer for hemophilia A and B. Three trials, each using different vectors and target tissues, are presently underway, and two additional trials are in late planning stages. This report reviews the preclinical data underlying these strategies and the design of the ongoing and proposed clinical trials.