Gene therapy is now being applied to the treatment of a wide variety of acquired and inherited diseases. One of the rate-limiting steps for successful gene therapy is the efficiency of gene transfer. A number of different viral systems are being developed for use as vectors for ex vivo and in vivo gene transfer, including retroviruses, adenoviruses, herpes- simplex viruses and adeno-associated viruses. These vital vectors have a number of specific advantages and disadvantages that make them suited to particular gene-therapeutic applications. This review will summarize the current status of the development of viral vectors for gene therapy.