As a result of repeated experiences with devastating earthquakes, storms, floods, and wildfires, places like Tokyo, Mexico City, San Francisco, and Los Angeles are already identified with catastrophe in both scientific literature and popular culture. Similar prospects face less obvious urban candidates like Dhaka, Miami, London, Lima, Seoul, and Sydney. In this collaborative study of environmental risks in ten of the world's major cities, geographers, planners, and other experts examine the hazard experiences of case study cities and analyze their future risks. They conclude that the natural disaster potential of the biggest cities is expanding at a pace which far exceeds the rate of urbanization. In addition to tracing hazard trends and arguing in support of management reforms that can be implemented quickly, Crucibles of Hazard directs attention to long-term issues of safety and security that must be resolved to sustain urban areas. Opportunities for such innovative policymaking include: capitalizing on the role of hazards as agents of urban diversification; broadening the scope for employing hazard-based contingency planning models in other urban governance contexts; and mobilizing hazard myths and metaphors as unifying sources of inspiration for diverse and sometimes fractious metropolitan constituencies. This study was led by the International Geographical Union's Study Group on the Disaster Vulnerability of Mega-cities.