Adeno-associated Virus (AAV) is classified as a member of the family Parvoviridae (Siegl et al. 1985; Berns 1990a). Members are small, nonenveloped, icosahedral viruses (diameter ca. 20–26 nm) with linear, single-stranded DNA genomes of 4.7–6 kb. Parvoviridae have been isolated from many species ranging from insects to humans. AAV is assigned to the genus Dependovirus, so named because the virus was discovered as a contaminant in purified adenovirus (Ad) stocks and in most instances does not productively infect cells in culture unless there is a coinfection by an unrelated helper virus, which is most commonly Ad, but also can be any type of herpesvirus (Atchinson et al. 1965; Hoggan et al. 1966; Buller et al. 1981). Various serotypes have been isolated from birds and many mammalian species, including humans. About 90% of U.S. adults are seropositive, but in no case has the virus been implicated as the etiological agent for a human disease or as the cause of disease in any other species. Because of the requirement for a helper coinfection for productive infection in cell culture, AAV was long considered to be a defective virus. Detailed studies described below have demonstrated that the virus is not defective, but rather preferentially establishes a latent infection in a healthy cell and is only induced to undergo productive vegetative multiplication when the host cell is stressed.