Transmission of plant viruses by their arthropod vectors is mediated by specific interactions between the plant, virus and vector. Most plant viruses are transmitted horizontally by their arthropod vectors, although reoviruses, rhabdoviruses and tenuiviruses may also be vertically transmitted. This chapter highlights recent progress and longstanding questions in vector biology. Bacterial endosymbionts of vector insects play a role in vertical transmission of plant reoviruses. Non-circulative virus transmission refers to viruses that are transiently and reversibly retained on the cuticle which lines the inner surface of the vector mouthparts. The best-characterized virus transmitted using the capsid strategy is the type member of the genus Cucumovirus, Cucumber mosaic virus, also renowned for its wide host range. Circulative virus transmission is characterized by viral entry into the insect body at a specific region of the alimentary canal, following acquisition during insect feeding.