Public health surveillance: what is it? Public health surveillance aims to describe a population health event to estimate its burden based on its characteristics (incidence, prevalence, survival and mortality) and their evolutions. This monitoring helps to contribute to the definition, implementation, monitoring and evaluation of public health policies. It must also be able to alert with the emergence of unexpected health phenomena. Public health surveillance is based partly on specific and continuous health information systems on populations (such registries), determinants or pathologies that are chosen a priori to monitor. It also relies on the collection of data, often in near real time (such SurSauD) and the construction of indicators a posteriori based on emerging health events and needs, or monitoring priorities. These two approaches are highly complementary. The contribution of these information systems (and of the health professionals who have to fill them out) is essential to the development of public health policies.