Eight Longidorus, one Paralongidorus and 7 Xiphinema species are natural vectors of nepoviruses and 7 Paratrichodorus and 4 Trichodorus species are vectors of tobraviruses. Nematode transmitted viruses and their vector nematodes are restricted in their distribution to regions with temperate or Mediterranean climates with the natural distributions of the viruses reflecting that of their vectors. In Europe the occurrence of a virus and vector frequently are localized within a relatively small area whereas in North America the viruses and/or their vectors are widespread. Specificity of transmission is indicative of a long association between virus and vector which has been determined and maintained by the range of factors influencing the complex nematode-virus-plant interactions. The influence of several ecological factors which may affect these interactions and the geographic distributions of the nematode/virus associations are reviewed and discussed.