The French Institute for Public Health Surveillance (InVS) has been entrusted by law with an increasingly prominent and urgent mission: to monitor the health of the population and alert public authorities in the event of risks or threats to public health. To fulfill this mission, the InVS has developed both methods in the fields of epidemiology and risk assessment, and a network of highly diverse partners. It works in concert with other health agencies, whose missions are all necessarily aligned with those of the InVS. Surveillance systems must be based on predefined action plans and take into account both feasibility—as determined by healthcare professionals—and acceptability—as perceived by the affected populations. The results must enable the analysis of evolving trends in diseases and exposures with a view to evaluating public policies. Surveillance must now extend, at the national and international levels, to all events that could pose a health threat. Expertise regarding these signals must enable the anticipation of such events. However, improving the population’s health cannot always be based on scientific certainties regarding risk management. Public health decisions are sometimes made in a highly charged context. There is often a disconnect between the calculated risk for a population (which differs from the actual risk for each individual) and the risk perceived subjectively by each individual or social group.