The identification of a case of childhood lead poisoning requires the physician to report the case to the state health authorities and initiate an environmental investigation. The purpose of this investigation is to identify individual sources of lead exposure. Although the prevention of lead poisoning relies primarily on the elimination of sources of lead, the environmental investigation is a valuable tool for the prevention, management, and eradication of lead poisoning, thereby addressing the priorities of environmental health policies through organized plans at various territorial levels (national, regional, city-wide, etc.). An initial guide was published in 2006 by the Institut de veille sanitaire (which became Santé publique France in 2016) to address the needs of services conducting investigations into cases of childhood lead poisoning. Following numerous developments, particularly the lowering of the mandatory reporting threshold on June 17, 2015, to 50 µg/L, it became necessary to update this guide. This update takes into account the new reporting threshold for lead poisoning cases, as well as newly identified sources of lead exposure. It outlines the general structure of the investigation and provides details on: Visits to various living environments, including the home as well as childcare facilities, schools, or out-of-home recreational spaces; Methods for sampling, collecting, and analyzing lead, and interpreting the results; The course of action to take in the event of an unsuccessful investigation; Available prevention tools; And procedures for reporting and summarizing the investigation results. Furthermore, it provides specific details on particular cases that may be encountered and the possible follow-up actions to be taken. It also notes that the environmental investigation, accompanied by specific questions about lifestyle, may be perceived by the family as an invasion of their privacy. The investigation must be conducted in a way that fosters an atmosphere of open communication and trust and is not perceived by the family as making them feel guilty. Initial prevention advice can be explained to the family during the first visit and at each stage of the investigation, depending on the suspected sources of exposure. This update is intended for both professionals experienced in conducting environmental investigations and those encountering them for the first time. It thus brings together the scientific and technical knowledge necessary for conducting these investigations while maintaining the document’s practical focus. To facilitate quick reading, a summary of the main points to remember is provided at the beginning of each chapter or subchapter. A summary of the investigation steps is also outlined in the conclusion.