Several highly virulent, arthropod-borne viruses (arboviruses) that cause serious human neurological diseases, such as West Nile encephalomyelitis, are transmitted by mosquitoes that feed on both wildlife and human hosts. The recent emergence of these viruses has prompted investigation into those mosquito species and wild birds that are conspicuously involved in transmission cycles. This chapter describes an innovative molecular technique used to identify the source of mosquito blood meals, which is proving to be quite useful in discerning the roles of particular vector and host species. This technique is used to determine the vectorial capacity of different mosquito species. Because species interactions are of prime importance in determining vectorial capacity, an understanding of community ecology is clearly essential for elucidating the epidemiology of vectored diseases.