Most plant viruses are transmitted by vectors from one host to another. Virus transmission by a vector is often characterized by some degree of specificity. Numerous studies suggest the involvement of a virus-ligand interaction in transmission specificity. The coat protein (CP) and its derivatives (readthrough CP and minor CP), and nonstructural proteins, such as a helper component (HC) or a transmission factor, are major viral determinants of transmission specificity. A number of virion- binding vector proteins have been identified as potential receptors. This article reviews the literature on the molecular aspects of virus transmission with a major emphasis on the specificity of transmission. It highlights recent advances in the field and identifies areas of desirable progress