Just as traditional environmental concerns-water, desertification, balanced development-can pose challenges to human security, so can the built environment. For the first time in history, the majority of people now live in cities, and rapid urbanization is already shaping trends in global peace and security. Armed violence is increasingly taking place in sprawling hillside slums, involving adolescent boys with automatic weapons, corrupt police officers determined to "clean up" city streets, or vigilante groups who take justice into their own hands. The violence feeds on the toxic mix of transnational criminal organizations and failed public security. But the built environment can also be a starting point for conflict resilience. Secure cities-cities with effective public security; inclusive, participatory governance; and positive social capital-will be critical to the prevention of armed violence and the protection of civilian populations from such violence when prevention fails.