This second section is devoted to local environmental health investigations. It was drafted by a working group comprising representatives from the agency’s various departments and volunteer members of the Steering and Dialogue Committee (COD). This field of environmental health has several distinctive features: the situations giving rise to complaints vary widely, the pathologies involved are multifactorial, environmental exposures are numerous, the latency periods between exposure and the onset of health effects can be long, and, from an etiological perspective, there are still many scientific uncertainties. The difficulty in reaching conclusions regarding causation leads to widespread misunderstandings and tension between experts and stakeholders. These situations cause anxiety, and even suffering, among the residents and families affected. They require the involvement of various stakeholders in contact with the public (healthcare professionals, elected officials, local governments, associations, etc.). Such situations often resonate from one region to another, and their impact can be significant at the national level. This document outlines the various stages of the participatory approach to be developed. Initially, an initial analysis of the context in which the complaint arose, based on interviews with a few key stakeholders, is recommended. If this context is deemed sensitive based on defined criteria, a more in-depth analysis of the questions, expectations, positions, and local actions is recommended. This will help identify the public health investigations best suited to address this request, as well as pinpoint differences in perspective among local stakeholders. On this basis, Santé publique France and other institutional experts will provide the most appropriate responses. This phase will also help identify the stakeholders to be involved in the committee process and precisely sequence the steps of the participatory framework, the principles of which were outlined in the first volume of the guide. A series of practical fact sheets concerning the identification and mobilization of stakeholders, as well as the concrete procedures for conducting local context studies, is presented. This guide is intended for Santé publique France teams, but may be useful to a wider audience.