This chapter addresses the ways in which water is allocated among different users at scales ranging from farmers sharing an irrigation ditch to states sharing a large transboundary river. We discuss economic theory, legal principles, and the complex regulations that govern water use. We address questions such as these:

• How should we allocate water among different users—and how do we actually allocate it?
• How have various communities managed to share water and avoid the tragedy of the commons?
• How do different US states allocate surface water and groundwater?
• What are tribal reserved rights and interstate water allocations, and how do they fit into the overall picture of water rights?