Since its creation in 2016 and as part of its activities, Santé publique France has been implementing an increasingly effective strategy of outreach to society. This openness is reflected in the agency’s governance, notably through the Steering and Dialogue Committee (COD), composed of citizens from civil society, and was formalized in 2016 with the signing of the Charter of Openness to Society, which brings together eight public organizations dedicated to research, expertise, and the assessment of health and environmental risks. The establishment of close relationships with the public, their representatives, and various stakeholders affected by the agency’s public health initiatives has also been firmly established for many years, both in the fields of prevention and health promotion as well as in epidemiological investigations. This experience is rich and diverse, involving various approaches that need to be better structured and shared, as demonstrated by an internal study published in 2016 on four participatory initiatives. This is the aim of this “Guideline,” which presents methods, tools, and procedures intended for Santé publique France staff and their partners when they plan to involve affected populations in their studies and surveys in general (Section 1) or more specifically in those concerning local environmental health investigations (Section 2). This first section outlines the general principles of participatory approaches, with semantic clarifications on several key concepts. The various forms of participation (information followed by discussion, consultation, deliberation, co-creation) are presented and illustrated with examples implemented by the agency in recent years. It then outlines the steps required to prepare for and conduct participatory processes, as well as to evaluate their impact on the study’s content, its reception, and the dynamics of relationships with stakeholders. Practical guides are made available to Santé publique France staff and their partners to facilitate the adoption of the various elements presented.