The North American Great Lakes constitute the largest surface of fresh water on Earth (Lakes Superior and Michigan-Huron alone are the two largest lakes on Earth by surface area). Monitoring and modeling the major components of the Great Lakes water budget, including over-lake precipitation, over-lake evaporation, and runoff, involves an international, multi-institution partnership that leverages a complex combination of sensor networks and modeling platforms. In this presentation, we provide an overview of the drivers behind long-term changes in Great Lakes water levels, including findings from recent research focused on explaining the abrupt water level decline on Lakes Superior and Michigan-Huron in the late 1990s, and the recent record-setting water level surge. Insights from this research underscore the sensitivity of large freshwater systems to regional climate perturbations, and the need for improved understanding of how the future of these systems will be dictated by a combination of climate change, human intervention, and changes in consumptive use.