A heat wave of exceptional intensity struck France in August 2003. The studies undertaken by the InVS beginning in August and the surveillance and warning systems subsequently put in place form the five chapters of this report. The health impact of the August 2003 heat wave was significant. This exceptional event raises the question of how to anticipate phenomena that are difficult to predict. The collaborations developed and all the actions and studies urgently implemented by the InVS will inform discussions on the implementation of warning strategies and, consequently, effective prevention. (R.A.)