Because of a reported excess of cancers among children living near power lines, there is some concern that electric and magnetic fields (EMFs) induced by electric power sources may affect human health, and this possibility has provoked considerable controversy. The scientific question of whether there are such health effects is far from resolved. Building upon a set of detailed reviews of the available evidence, this paper proposes research priorities and places particular emphasis on epidemiologic research. The most pressing need is to verify the validity of the claim that childhood cancer risk is affected by the type of wiring code in the vicinity of the household. More useful work can be done to verify this in the areas in which such studies have already been carried out, and additional studies should be done elsewhere. Methodological investigation of the interrelationships among different measures and proxies for EMF is needed, and this could feed back to influence the type of EMF measures used in epidemiologic studies. Studies of cancer among adults in relation to EMFs in the work place are needed. Of lower priority are studies of adverse reproductive outcomes in relation to parental EMF exposure and studies of the neurobehavioral impact of chronic EMF exposure. This article also discusses the structural impediments of conducting environmental epidemiology research and argues that bold, large-scale epidemiologic monitoring systems are needed. There is a discussion of the interface between epidemiology and public policy in a topic area as controversial as EMFs.