More than just an investigation guide, this document is intended as a resource dedicated to the detection and investigation of outbreaks of infection linked to the consumption of drinking water. It also includes recommendations regarding surveillance and research needs in this field. Operational information is compiled in a separate document available on the InVS website.The drafting of this document was based, on the one hand, on a critical review of outbreak investigations conducted in France since 1998, and on the other hand, on the expertise of a working group composed of epidemiologists, sanitary engineers, and microbiologists. The various stages of the investigation (detection, exploratory investigation, and in-depth investigation) are detailed by discipline (epidemiology, sanitary engineering, and microbiology). The synergies expected from interdisciplinary collaboration, particularly in the early phase of the investigation, are highlighted, and collaboration among stakeholders is encouraged. Analysis of the current situation shows that public health progress is possible both in preventing cases during outbreaks that have already begun and in preventing outbreaks altogether. The main drivers of progress are improving the comprehensiveness and timeliness of detecting outbreaks or situations at risk of an outbreak.Currently, the signals triggering outbreak investigations come primarily from regulatory water monitoring and the identification by field personnel (doctors, facility managers, members of the public) of clusters of gastroenteritis cases. Signals arising from water system operations (accidental contamination, treatment failures) and consumer complaints, however, are largely underutilized. Proposals have been made to better integrate information held by water system operators into the process of detecting and preventing outbreaks. Health insurance medical billing data could also be utilized online to detect an excess incidence of cases at the municipal level. An experimental study is currently underway at the InVS to determine the usefulness of this source for detecting waterborne gastroenteritis outbreaks.The authors are aware of the document’s shortcomings, particularly the insufficient specification of signals derived from water management that are useful for predicting epidemic risk, such as those associated with turbidity events, chlorination failures, or pipe breaks. With a view to a second edition, the authors are opening a dialogue with local partners willing to share their experience, such as the DDASS and water operators. (R.A.)