Publisher Summary This chapter discusses the Arboviruses. The Arbovirus group is the largest of the recognized virus groups and comes second only to the Picornaviruses in order of clinical significance. It is not a homogenous group and will probably be divided when the basic properties are adequately determined. An Arbovirus has been defined as the one, which in nature, can infect hemophagous arthropods by their ingestion of infected vertebrate blood. It multiplies in their tissue and is transmitted by bite to susceptible vertebrates. There are approximately 170 known types divided into about 21 subgroups. Like all Arboviruses, Yellow Fever Virus is not only transmitted by an arthropod but also multiplies within the insect. Although, the Arboviruses are restricted to the areas of their vectors, no part of the world is entirely free of them because the vectors can be mosquitoes, ticks, sandflies, or midges. The host range of the Arboviruses is the widest of all the viruses, encompassing mammals, birds, and arthropods. The ungrouped viruses number at least 50 and only those which are important to medicine or veterinary medicine.