The unifying feature of arbovirus biology is the development of complex arthropod-vertebrate virus transmission/maintenance cycles. In such cycles, virus genetic information must be successfully expressed in two phylogenetically unrelated biological systems. Elucidation of the evolutionary patterns of arboviruses is important. Genotypic changes, which result in altered phenotypes, could exhibit enhanced virulence or altered tropisms in vertebrates. Resultant antigenic changes could also provide a mechanism for viruses to elude existing immunologic barriers in the vertebrate populations, resulting in serious epidemiologic consequences. One can only speculate concerning the epidemiologic implications of arbovirus evolution. Dramatic changes in the epidemiology of certain arboviruses may or may not be attributable to evolutionary events. There are examples of disease outbreaks or altered arbovirus epidemiology in which virus variation could have played an important role.