Epidemiological surveillance is one of the responses that can be provided to local requests regarding environmental health. Although it does not answer all questions, it is often cited by communities and decision-makers concerned about the health consequences of environmental exposure. The study presented here, conducted at the request of the Directorate General of Health, focused on the process for assessing the relevance of such surveillance, defining its scope, and developing implementation protocols, based on an analysis of a range of situations (polluted sites and soils, classified facilities, nuclear activity zones, post-accident situations) where the issue of surveillance had arisen or continues to arise. This activity, deeply rooted in the local context, closely aligned with the definition of epidemiological surveillance, while highlighting certain features such as the expert’s interaction with local stakeholders or the variety of methods through which it is carried out (ranging from the conventional use of medical-administrative databases to the analysis of data from screening or medical follow-up). The path leading to the decision to establish a surveillance system was described and analyzed in each of the reported cases, using the notion of relevance (conceptual and practical). This led to the development of a set of decision-making tools (including arguments justifying the need for surveillance, and health and/or environmental conditions that help assess its relevance and nature, among others). These tools are not organized in the form of a guide. The diversity of environmental situations, challenges, and public perceptions made it impossible for the authors to identify a “generic” course of action based on the various approaches analyzed. Therefore, as its title suggests, this document instead offers a set of elements intended to enable each individual to ask the appropriate questions and organize the answers to them in order to assess, on a case-by-case basis, whether or not it is appropriate to establish a local monitoring system. This work should be continued on a forward-looking basis. Although it cannot be ruled out that an increase in the number of analyzed situations will merely highlight new specific cases, it is also hoped that the exchange of knowledge and expertise may lead to less disparate approaches in the future, thereby establishing a more consensual course of action, while resolving methodological issues related to local studies that remain under debate to this day. This work will also benefit from a more comprehensive approach, addressing the various ways (including surveillance) to respond to local requests regarding environmental health. (R.A.)