When an arthropod feeds on a viremic host, there is potential for the arthropod to transmit the virus either mechanically or biologically. In the former case, the vector need not be susceptible to viral infection; while in the latter, virus must replicate in the vector before the virus can be transmitted. Once it has replicated in the vector, the virus may be transmitted either horizontally to a vertebrate by bite, or vertically to the vector’s progeny by transovarial transmission. Transovarially infected individuals then have the potential to retransmit the virus vertically to their progeny, or to transmit it horizontally either by bite to a vertebrate or venereally during mating. This chapter discusses various factors that have been shown to affect the efficiency with which arboviruses are transmitted by their potential vectors. While many of the principles of arbovirus transmission apply equally to tick or insect transmission cycles, there are some inherent differences.