Large cities are particularly vulnerable to heat waves due to their population density, land-use characteristics, anthropogenic heat generation (the urban heat island effect—UHI), and air pollution. Gaining a better understanding of the determinants of vulnerability to heat waves to guide prevention efforts is all the more important given that, in the context of climate change, observations and models of global temperature trends indicate an increase in the frequency, intensity, and duration of heat waves in Western Europe. The InVS has launched several studies to analyze the impacts of the August 2003 heat wave. Among them, a case-control study aimed to examine risk factors for mortality among older adults living at home, with a particular focus on environmental factors, including temperature. In fact, of the 15,000 people who died during the first half of August, 91% were aged 65 and older and lived in urban areas, and 35% died at home. This study was repeated in 2008–2011 through a collaboration with the CNRS “Géomer” laboratory at the University of Western Brittany (a project funded by MAIF), using new environmental indicators derived from satellite imagery. (R.A.)