Adaptive management represents the future of natural resource management, including that of water and aquatic resources. Adaptive management is an inherently flexible system, in which resource managers establish desired outcomes, develop hypotheses and monitoring programs to test whether existing management approaches are achieving those outcomes, and then alter the approaches depending on the monitoring results. The need for adaptive management will become even more acute as arid region water resource managers struggle to cope with the additional uncertainties in water supply expected to result from global warming. In order to fully implement principles of adaptive management, resource managers need to cope with change and uncertainty, and need flexible management tools at their disposal. Yet this needed flexibility is incompatible in significant ways with existing water law in arid regions such as the western United States and other arid countries around the world. This Article addresses legal and policy reforms needed to adapt arid lands' water law to the goals and practices of adaptive management.