Polydnaviruses (PDV) are symbiotic viruses of certain parasitic Hymenoptera, and have been found only in Ichneumonidae and Braconidae. These viruses are assigned to the family Polydnaviridae which consists of two groups, ichnovirus and bracovirus. Polydnavirus genome is made up of multiple superhelical closed circles of double-stranded DNA. Viral replication is restricted to the calyx epithelial cells of parasitoid ovary, and the PDV can be transmitted vertically in the form of an endogenous provirus during parasitization. These viruses can inhibit the host immunosystem, which allows the parasitiod to develop in host without encapsulation and melanization, and modifies the physiology, growth and development of their hosts. The influences of polydnavirus on host immunity and endocrinology have attracted much academic attention in the last decade. Detailed research progress on polydnaviruses and their functions as mutualists and pathogens is reviewed. none