City life is becoming the default human experience. Cities are engines for development, and increasing urban populations provide connection and opportunity. But cities can also exacerbate some of the world’s most serious environmental and socioeconomic challenges. Cities concentrate millions of people into locations that can be highly vulnerable to disaster, pollution, and the impacts of climate change. Twenty-one of the world’s 33 megacities are in low-lying coastal areas: it is estimated that almost 700 million people live in urban or peri-urban areas that are less than 10 meters above sea level. Meanwhile, cities are hotspots for both air pollution and disease. A recent study estimated that meeting WHO air quality standards in 1,000 European cities would save 52,000 lives annually.