Ground water is a source of drinking water for many people and is the primary source for irrigation and livestock watering in the Great Lakes region. The use of ground water in the Great Lakes Basin has substantially increased in the past few decades due to population growth, technological innovation, agricultural development and inefficient water use. Despite the increase in demand, there have been no significant changes in the ground water allocation policies in either Canada or the United States since the nineteenth century. Six of the ten jurisdictions of the Great Lakes Basin still rely on archaic common law principles to determine the allocation of ground water, while Ontario's water taking permit program has shown that centralized government regulation can be equally ineffective. Therefore, the courts and governments of the Great Lakes Basin are effectively encouraging unrestricted withdrawals of ground water, and as a result, water tables are declining, well interference incidents are increasing and ground water divides are shifting. These physical effects are giving rise to economic costs, social conflicts and environmental degradation. To mitigate the impacts of antiquated ground water allocation policies in the Great Lakes Basin, the authors suggest institutional change and a range of legal tools to better protect this critically important resource.