Many important plant pathogens are viruses, which can cause severe losses in vital crops. Indeed, viruses constitute the largest group of emerging or novel plant pathogens. Nevertheless, while it may appear paradoxical a growing evidence base indicates that plant viruses can confer benefits on their hosts that may increase host survival and reproductive success. These apparently mutualistic effects may be conditional, for example, by aiding host survival under conditions of drought or cold or, more surprisingly, by helping plants attract beneficial insects such as pollinators. Such effects are thought to be able to influence the evolution of both the host and virus. Certain plant viruses including ones belonging to the Partitiviridae and Endornaviridae families seem to have travelled furthest on an evolutionary path towards becoming beneficial plant symbionts. Sometimes called plant ‘persistent’ viruses because they are transmitted exclusively via seed and pollen at rates of up to 100%, these viruses are not infectious in a conventional sense (i.e., not transmitted by vectors, wounding, or grafting), these agents do not appear to induce any obvious symptoms. It has been hypothesized that plant persistent viruses are mutualists. Recent work testing this hypothesis will be discussed.