Climate change and its consequences will affect populations worldwide. Extreme weather events, such as heat waves, cold spells, heavy rainfalls and droughts, are expected to become more frequent and more intense. Also, Austria will be increasingly affected by changing climatic conditions, particularly with respect to thermal hazards. The „Climate, Health and Population – Climate Change and Differential Vulnerabilities in the Metropolitan Area of Vienna“ (CHAP) project, funded by the Jubilee Fund of the City of Vienna for the Austrian Academy of Sciences, studies the impact of changing environmental conditions on population health in the metropolitan area of Vienna. A particular focus will be placed on the effects of thermal hazards, which are predicted to become more severe, especially in fast-growing cities like Vienna. The project has two main objectives: First, we analyze the influences of temperature extremes, i.e. heat waves and cold spells, on morbidity and mortality, using historical data on hospitalizations and medical treatments. Second, building on the findings from the first part of the project, we project future climate and population dynamics and estimate the expected health burden for the metropolitan area of Vienna. As climatic and demographic changes can mutually reinforce each other, taking an integrated perspective is essential. As further novel contribution, we do not only take social factors as drivers of differential health vulnerabilities into account, but also structural aspects of the built environment, such as urban density or the availability of green spaces, which can help mitigate the harmful effects of weather extremes. The expected insights from the project are of high relevance both for academic research and policy makers and have multiple implications for urban planning and public health.