Urbanization continues to increase rapidly on a global scale. Today, more than four billion people live in cities; by 2050, this number is projected to double, and nearly 7 in 10 people will live in urban environments. Cities are nexuses of economic and social activity and hubs for knowledge and culture. As they expand, so does their capacity—and their responsibility—to foster environments in which people can thrive. Cities hold tremendous, often untapped potential to improve health: the proximity of people to services and infrastructure means easier access to health facilities and other essentials, population density can support economies of agglomeration which lower costs, and the concentration of human ingenuity fosters technological innovation to support health improvements. However, cities can likewise give rise to significant health and environmental issues: overcrowding, air pollution, urban waste, and insufficient social protection, among other urban determinants, can lead to disease transmission, logistical difficulties in care provision, health disparities, chronic illnesses, and more. The breadth and seriousness of such urban health challenges tend to be greater in developing countries, but significant problems also exist in cities of the Global North.