Arid regions and their cities are vulnerable to future water scarcity because climate change threatens to reduce supply and rapid growth increases demand. The study of water sustainability in these regions and cities transcends concern about climate change and growth, however, and includes the dynamics of water–energy relationships, tradeoffs involved in the use of irrigated landscaping for temperature amelioration, and feedbacks between urban growth, the economy, and the environment. It requires analysis of how complex human and biophysical systems function at a range of scales and calls for new tools for risk assessment and decision support incorporating decision making under uncertainty.