Eastern equine encephalomyelitis virus (EEEV) has been the cause of epizootics of eastern equine encephalitis (EEE) in North American horses as far back as 1831, but undoubtedly the virus itself was present in its endemic cycle long before that. Mosquitoes were first incriminated as potential vectors of EEEV in 1934, and a number of studies have shown species of Aedes, Culex, and Coquillettidia could become infected with and transmit EEEV from one vertebrate to another. The social and economic impact of mosquito control for EEE prevention is considerable once the potential for disease is recognized in an area. Economic losses have been documented for the 1959 outbreak in New Jersey. Vaccines have proven to be very effective for preventing equine EEE, but recent evidence suggests that there are windows of susceptibility of foals in current vaccination practices.