Eighteen species in the plant-parasitic nematode genera Longidorus, Paralongidorus and Xiphinema are vectors of twelve nepoviruses and in the genera Paralrichodorus and Trichodorus thirteen species are vectOrs of all three tobraviruses. A characteristic of these vectOr nematOde and virus associations is that serologically distinct nepoviruses and virus strains are transmitted by different, but related, longidorid species. The viruses are referred tO as having "specific" vector species, and the terminology has been adopted by researchers who refer to "specificiry" of transmission of viruses by vector nematodes. Further research has confirmed that specificiry of transmission extends to populations of vector species and to minor serological variants of nepoviruses. Ir has a1so been shown to extend to tobraviruses and their vector species. We describe the possible mechanisms which determine the association between veetor nematOdes and their viruses and provide and explain definitions of several terms and concepts used in veetor nematode research.