Lake Ontario has profoundly influenced the historical evolution of North America. For centuries it has enabled and enriched the societies that crowded its edges, from fertile agricultural landscapes to energy production systems to sprawling cities. In The Lives of Lake Ontario Daniel Macfarlane details the lake’s relationship with the Indigenous nations, settler cultures, and modern countries that have occupied its shores. He examines the myriad ways Canada and the United States have used and abused this resource: through dams and canals, drinking water and sewage, trash and pollution, fish and foreign species, industry and manufacturing, urbanization and infrastructure, population growth and biodiversity loss. Serving as both bridge and buffer between the two countries, Lake Ontario came to host Canada’s largest megalopolis. Yet its transborder exploitation exacted a tremendous ecological cost, leading people to abandon the lake. Innovative regulations in the later twentieth century, such as the Great Lakes Water Quality Agreements, have partially improved Lake Ontario’s health. Despite signs that communities are re-engaging with Lake Ontario, it remains the most degraded of the Great Lakes, with new and old problems alike exacerbated by climate change. The Lives of Lake Ontario demonstrates that this lake is both remarkably resilient and uniquely vulnerable.