In this paper, we have proposed a wider scope for public health surveillance in order to incorporate demographic and health-system monitoring, along with activities conventionally associated with epidemiologic surveillance. This new conception stems, in turn, from a revised definition of public health, which describes, not a sector of activity or a type of health service, but a level of aggregation based on the population at large. In our review of the ideas that lead to the institutionalization of health surveillance, we have stressed the broad concepts developed by such pioneers as Graunt and Petty. Their original concepts emerged from their active concerns for the public's health at a time when no scientific theory of contagion was available, let alone any knowledge about how to treat persons for the major diseases. Later on, largely as the result of impressive advances in biomedical knowledge, public health surveillance tended to specialize and to concentrate predominantly on disease outbreaks and on salient adverse health conditions. Health surveillance became closely associated with epidemiologic surveillance, which in turn became associated with the ability to respond promptly to adverse health outcomes. Recently, we have witnessed a gradual broadening of both the concepts and the practice of health surveillance. Paradoxically, the new currents tend to recapture some of the spirit and scope of the early definitions, prompted perhaps by grave historical parallels--we face newly emerging health problems for which we have no clear-cut solutions. If one element needs to be stressed to promote the objectives of health surveillance today, it is that we need the ability to anticipate health outcomes and not just respond to them. This, in turn, requires that we give more weight to the surveillance of risk factors and that we increase our understanding of the complex causal interrelationships that link exposure to risk factors--including behavioral, life-style, and environmental ones--with adverse health conditions and disability. Needless to say, the first and foremost aim of health care--and modern surveillance is one of the tools needed to achieve this aim--is to promote the well-being of individuals while improving their health.