Hemophilia A and B are X-linked bleeding disorders caused by mutations within the factor VIII and factor IX genes, respectively. Although both disorders can be easily treated by substitution with concentrates of functional factor VIII and factor IX, considerable effort has been undertaken to develop a gene therapy for hemophilia in order to improve patients' life quality and reduce high costs of therapy. The principle of gene therapy is the introduction of an intact copy of the factor VIII/factor IX gene in somatic cells, compensating for the defective gene. To do this, retroviral, adenoviral, and adeno-associated virus (AAV) vector systems, among others, were used. Encouraged by the results of preliminary experiments using preponderant mouse and canine models, three clinical phase I studies on hemophilia A and B patients have been initiated, one of which has been preliminarily reported successful.