none Symbiotic associations traditionally have been treated as evolutionary curios rather than as a major source of evolutionary innovation. Recent research on a wide variety of organisms is changing this view and is breaking down the barriers between the traditional categories of parasitism, commensalism and mutualism, to produce a more flexible view of multispecific interactions. An especially abundant, but little discussed, mutualism exists between parasitoid wasps in the superfamily Ichneumonoidea and a novel form of DNA viruses known as polydnaviruses. Mutualisms between viruses and eukaryotes are not often reported, although as many as 100,000 species of organisms may exhibit this unusual association. In this review Jim Whitfield considers what is known about the parasitoid-polydnavirus relationship and how (and from what) it might have arisen. Nutrition; Endocrinology; Associations; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Diseases and disorders; Diseases; Microbial diseases; Transmission; Immunology and repair mechanisms Insecta; Nutrition; Endocrine system; Viral disease effects, influence on host/hymenopteran parasite relations; Hymenopteran parasites; Host parasite relations, role of parasite symbiotic virus; Host parasite relationships; Viral disease relations after transmission by hymenopteran parasites; Viral diseases; Physiological effects, transmission by hymenopteran parasites; Transmission of viruses; Polydnavirus; Transmission by hymenopteran parasite, role in host parasite relations; Immune like reactions; Suppression by viral disease, hymenopteran parasite relations Hymenoptera; Mutualism; Virus role in insect host/parasite relationships; Insect hosts; Host parasite relationships & role of symbiotic viruses; Host parasite relationships; Symbiotic viruses role in insect hosts, review; Transmission of viruses; Polydnavirus; Transmission to insect hosts, role in host parasite relations