This document compiles recommendations for the preparedness (including improved detection) and investigation of outbreaks of gastrointestinal diseases caused by the ingestion of drinking water (from public, private, or military water systems). This excludes outbreaks linked to private wells, swimming, or other water-related activities, as well as outbreaks of skin or respiratory diseases, particularly Legionnaires’ disease. The guide may nevertheless be useful for investigating outbreaks attributable to the ingestion of tap water involving other health effects, whether of infectious or toxic origin. It is primarily intended for staff at the Ddass and Cire who may be responsible for or involved in investigating outbreaks of infection linked to the ingestion of drinking water. It may also be useful to operators of drinking water production and distribution systems, mayors, local elected officials, and presidents of drinking water associations, as well as healthcare professionals (general practitioners, emergency physicians, and community pharmacists). It is based on the work of the working group dedicated to improving investigations of waterborne outbreaks (2002–2006) and, in particular, on the resource document “Detection and Investigation of Outbreaks of Infection Linked to the Ingestion of Tap Water: An Integrated Environmental and Health Approach,” available online on this site. Further details on the points mentioned in this guide, as well as bibliographic references and appendices, can be found in the resource document, also available online at the aforementioned website. This guide is subject to change. The version of the guide available on the InVS website is the most current version. (R.A.)