In the aftermath of a heat wave, the Department of Environmental Health launched a forward-looking monitoring initiative to identify environmental threats to public health with the aim of taking proactive measures. Under the supervision of a scientific steering committee chaired by the InVS and involving institutional and private partners, the study commissioned from HPC Envirotec aimed to identify and categorize environmental threats from a strategic perspective to inform the implementation of appropriate monitoring systems. It also enabled the development of a scalable method capable of integrating threat categorization as soon as threats are identified. Based on the most comprehensive bibliographic research possible, the work first involved defining an environmental threat as an exposure situation and then describing each of these threats in terms of a location, a vector, an agent, and a source. The categorization of threats was based on criteria for assessing their public health significance. This resulted in three categories, ranging from those already subject to a public health surveillance system to those not currently addressed by public authorities. The diversity of identified threats is vast; air and the built environment are the most frequently cited vector and location. The next step is to prioritize threats within each category and to compile experiences from systems that have been established or are currently being developed at the international level. (R.A.)