Environmental change and natural hazards represent a challenge for sustainable development. By disrupting livelihoods and causing billions of dollars in damages, disasters can undo many decades of development. Development, on the other hand, can actually increase vulnerability to disasters by depleting environmental resources and marginalizing the poorest. Big disasters and big cities get the most attention from the media and academia. The vulnerabilities and capabilities of small cities have not been explored adequately in academic research, and while some cities in developed countries have begun to initiate mitigation and adaptation responses to environmental change, most cities in developing countries have not. In this thesis I explore the vulnerability to flooding of the US-Mexico border by using the cities of Nogales, Arizona, USA and Nogales, Sonora, Mexico as a case study. I ask the following questions: What is the spatial distribution of vulnerability, and what is the role of the border in increasing or decreasing vulnerability? What kind of coordination should occur among local institutions to address flooding in the cities? I use a Geographic Information System to analyze the spatial distribution of flood events and the socio-economic characteristics of both cities. The result is an index that estimates flood vulnerability using a set of indicators that are comparable between cities on both sides of the border. I interviewed planners and local government officials to validate the vulnerability model and to assess collaboration efforts between the cities. This research contributes to our understanding of vulnerability and sustainability in two ways: (1) it provides a framework for assessing and comparing vulnerabilities at the city level between nations, overcoming issues of data incompatibility, and (2) it highlights the institutional arrangements of border cities and how they affect vulnerability.