The 2004 heat wave warning system was developed based on a retrospective analysis of health and meteorological data from fourteen pilot cities. The identification of a relevant biometeorological indicator among the seven indicators tested (based on temperature and humidity) was based on sensitivity and specificity criteria. The composite indicator, combining minimum and maximum temperatures above a threshold, was selected. The data were averaged over three days to account for the persistence of the heat. The selected thresholds allow for the anticipation of large-scale events resulting in excess mortality exceeding 100% in the smallest urban areas and 50% in Paris, Lyon, Marseille, and Lille. The geographic extension was performed using the 98th percentiles of minimum and maximum temperatures, which are very close to the thresholds of the pilot cities. The National Heat Wave Plan, which incorporates this system, is divided into four levels. Level 1 is a seasonal watch level. Level 2 is triggered when thresholds are expected to be exceeded with three days’ notice or less; Level 3 is triggered when thresholds are expected to be exceeded on the same day. Level 4 corresponds to an exceptional situation involving collateral damage. The system is operational from June 1 to September 30 and involves continuous coordination between Météo-France, the InVS, the Cire, and health authorities.