Two trends—urbanization and environmental degradation—are on a collision course. Already, over half of the world’s people live in cities, and more are added every day. At the same time, many cities are at risk of becoming uninhabitable, with air pollution the primary culprit. Substandard air quality levels affect more than 80 percent of people living in urban areas that monitor air pollution, according to the World Health Organization (WHO). The problem is particularly stark in low- and middle-income countries, where the air quality in 98% of cities does not meet WHO guidelines. On average, global air pollution levels rose by 8 percent from 2008 to 2013, bringing higher risk of stroke, heart disease, lung cancer, and asthma, among other diseases. Reversing this trend and making all cities livable spaces will require bold policy leadership to foster clean transportation, energy efficiency, renewable energy, and dense urban forms to counter auto-dependent sprawl. Such measures can foster a healthy population while helping to mitigate global climate change: a win-win agenda for people and planet.