Natural resource scarcities are fundamental challenges for cities – and water is an especially critical resource. The chapter examines how maintaining adequate water supply is a constant challenge for cities. More specifically, this application chapter focuses on how urban water supply can be maintained, and when and why threats to supply emerge and can grow. How urban water supply stress and crisis can lead to significant environmental policy transitions is the focus of four specific city case studies. These include Atlanta, US; Cairo, Egypt; New York, US; and Tel Aviv, Israel. While droughts were relevant proximate drivers of the transitions, the cases illustrate how a range of root drivers (e.g., lack of governance capacity, transboundary conflicts) and context drivers (e.g., competing economic interests, legal precedents, inequity) played significant roles in the policy transitions and resulting transformation. The social conditions, ecological constraints, and technology access all took on important functions. Built infrastructure, including water supply infrastructure (e.g., dams, reservoirs, aqueducts) as massive, fixed assets representing the legacy of past actions, was especially important in the articulation of the transition process.