Viruses are parasitic entities that have successfully evolved over millions of years due to their ability to introduce their own genetic code into the cells of their target hosts. They are capable of entering a host, passing the physical barriers that comprise tissue, infecting cells by active mechanisms and delivering their genetic cargo direct to the nucleus of a cell. As such they provide the perfect vehicle from which to deliver therapeutic DNA. One of the biggest hurdles to the widespread application of viruses in patients is the design of a safe attenuated virus that maintains its ability to efficiently transfer genetic material. Given that most viruses are associated with induction of significant pathology in humans, this is no easy task. Also, after infection, most viruses elicit a strong immune response that effectively removes virally infected cells. Despite of these hurdles significant progress has been made in the design of gene transfer vehicles based on attenuated viruses.