In response to the consequences of the exceptional heat wave of 2003, the National Heat Wave Plan (PNC) was developed in 2004. Updated in June 2005, this plan establishes an alert and monitoring system and a response framework for heat waves, outlining the actions to be taken in the event of a heat wave. Implementation of the warning and monitoring system, known as the Heat Wave and Health Warning System (SACS), was entrusted to the French Institute for Public Health Surveillance (InVS). The warning system was designed to alert public authorities three days in advance of the possible onset of a large-scale epidemic linked to a heat wave. Surveillance is based on two types of indicators: biometeorological indicators (IBM) on the one hand, and health indicators—mortality and morbidity indicators (IMM)—on the other. IMMs are collected regionally by the Interregional Epidemiology Unit (Cire) from various local partners serving as data sources. The 2005 National Heatwave Plan (PNC 2005) establishes continuous collection of these MMI data throughout the entire surveillance period, unlike the 2004 PNC, which defined MMI collection only during the alert phase. Thus, in 2004, for the Pays de la Loire region, the implementation of the SACS was tested for only one week, at the end of June, and 2005 marked the effective implementation of the SACS. Following feedback from the 2004 SACS season, the IMM thresholds were also revised. The purpose of this report is to first present the principles of monitoring, the regional organization adopted, the methodology used, and finally the results of the monitoring. (introduction)