Research leading to the current state of knowledge on the epidemiology of La Crosse virus (LACV), Jamestown Canyon virus (JCV) and dengue (DEN) viruses is summarized in relation to the generally recognized criteria for incriminating vectors. The importance of vector biology and local ecological conditions is emphasized as is the necessity of a good balance between laboratory and field-based studies. The influence of human activity in shaping the epidemiological patterns of all three of these arboviruses is readily apparent. [References: 157]