The Parvoviridae have been of increasing interest to reseachers in the past decade. Their small size and simple structure have made them ame- nable to detailed physiochemical analysis, and from this work relatively detailed information has resulted that has signficantly increased our un- derstanding of the biology of these viruses. It has become clear that the Parvoviridae are of interest not only for their own sake, but also because their relative simplicity renders them useful probes in the study of the biology of host cells and of other DNA viruses with which they interact. The Dependovirus genus, for instance, contains the defective adeno-as- sociated viruses (AA V), which require a coinfection with either an ad- enovirus or a herpesvirus for productive multiplication. Studies of AA V, therefore, necessarily impinge on our understanding of the control of macromolecular synthesis by the helper virus. Similarly AA V has been reported to inhibit the oncogenicity of both adeno-and herpesviruses and has been used as a probe of mechanism in these instances as well. Finally, AA V establishes latent infections in vivo and is the only mammalian DNA virus where a comparable model system has been established in cell culture. This system has allowed study of the mechanism of latent infection at the molecular level.