The purpose of this report is to present the 2005 version of the heat wave and health alert system. The system remains based on the same principles that were in place during its first summer of operation, namely that an alert is issued when two biometeorological indicators simultaneously exceed their respective thresholds. These indicators are the three-day moving averages of minimum (IBMn) and maximum (IBMx) temperatures, and the corresponding thresholds are defined for each department with the aim of identifying potentially risky days. However, the numerical values of the thresholds have been modified, and qualitative criteria to aid in the alert decision-making process have been added. In addition, certain practical aspects of the alert system’s organization have also been modified. This report first provides a summary of the Heat Wave and Health Alert System (SACS) as it was designed and implemented in 2004, focusing primarily on methodological aspects and presenting the main results. The various aspects of the evaluation of the 2004 heat wave alert system are then presented (external evaluation, European workshop, internal evaluation, work of the DGS-InVS and InVS-Météo-France working groups), as well as improvements compared to last summer: modification of certain reference weather stations, changes to the criteria for selecting biometeorological indicators and calculation of new thresholds, and definition of qualitative criteria (meteorological, environmental, social). Organizational aspects are discussed, particularly regarding decision-making procedures, information provided by Météo-France, the collection and analysis of mortality and morbidity indicators by the CIREs, relationships between partners (various departments and bodies of the InVS, DGS, Météo-France), and a summary of alert and information channels. The 2005 SACS is integrated into the new version of the National Heat Wave Plan, which is divided into four levels: seasonal monitoring, pre-alert, alert, and maximum mobilization. It is operational from June 1 to August 31 and involves continuous interaction between Météo-France, the InVS, the CIREs, and health authorities. (R.A.)