Abstract:Cities have become important actors in international relations, and integral to security and environmental politics. We are living in an increasingly urban world, dominated by human settlements and activities. The central role now played by humans in shaping the planet has led us into an uncertain, unstable, and dangerous geological epoch -the Anthropocene -that poses great and additional challenges to security. Local and global spheres are connected as never before, generating 'glocal' issues in which water plays a central role. Water is the element that interconnects the complex web of food, energy, climate, economic growth, and human security. In a rapidly urbanising world, cities influence the hydrological cycle in major but uncertain ways, affecting water resources beyond their boundaries. There is no doubt that these issues are highly relevant to the discipline of International Relations (IR). However, IR scholars have been slow to engage with them, and most academic studies of cities and water security still emanate from the natural sciences. This article examines the ways in which cities in the Anthropocene challenge water security, and why IR needs to reinvent itself if it wants to sustain its contribution to global security.