Dengue and dengue hemorrhagic fever are emerging as major public health problems in most tropical countries. Effective prevention and control programs will depend on improved surveillance designed to provide early warning of dengue epidemics. This article outlines a reasonable approach to dengue surveillance of this kind. Virologic surveillance should be considered the most important element in any such early warning system. Dengue virus transmission should be monitored to determine which serotypes are present, their distribution, and the type of illnesses associated with each. Other key components of an active surveillance system should include monitoring of fever activity and clinical surveillance for cases of severe and fatal disease associated with viral syndromes. Collectively, these three surveillance components can provide an early warning capability permitting emergency mosquito control measures to be implemented and major epidemics to be averted.