Technical Abstract: Aphid transmission of luteoviruses is often said to be vector specific, i.e. most luteoviruses are transmitted specifically by one or at most a few aphid species. The transmission specificity is rarely absolute and in most cases only refers to that aphid species which most efficiently transmits the luteovirus or luteovirus isolate. These virus-aphid associations are the most common based on published research, however, as additional luteovirus isolates are characterized it is apparent that heterogeneity among virus isolates and aphid species may alter the generally accepted transmission phenotype of many luteoviruses. Luteoviruses can be heterogeneous within a virus group or even within a serotype. The heterogeneity can be indentified in a variety of ways; serology, nucleic acid properties, disease reaction or in aphid transmission phyotype. The latter is especially true for the barley yellow dwarf luteoviruses. To complicate matters further, numerous studies indicate that there is also genetic heterogeneity of luteovirus transmission factors within an aphid species. This section will review the understanding to date on how genetic diversity withinan aphid species can effect changes in the transmission efficiency of luteoviruses.