After more than a century of steady city expansion in northern countries, the direction of twenty-first century population growth is shifting southwards. Over the next five decades, Africans, Arabs, and Asians will migrate in unprecedented numbers to cities, especially to their slums. Many of these urban settlements are insecure, disorganized, and violent. These are fragile cities and such migrations can threaten their inhabitants, countries, and the wider neighborhood. The analytical focus on fragile cities offers a novel scale when compared to fragile and failing states. It is also one that is preoccupying national policymakers, military strategists, and development experts. Drawing on theoretical and empirical contributions from geography, criminology, and sociology, this article identifies four mega-risks shaping urban fragility—the transformation and concentration of violence, turbo-urbanization, youth bulges, and the relentless penetration of new technologies. It also considers successful approaches to reversing city fragility, including twinning fragile cities with healthier and wealthier ones, investing in hotspot policing, interventions addressing at-risk youth, support for inclusive and cohesive urban growth, and the targeted application of new technologies.