It has become popular to argue that American energy security and climate change challenges can be solved by a series of "win-win" policies that tackle both problems at once. A look at the future of the Canadian oil sands calls that belief into question. Exploiting the oil sands would boost the world's supply of liquid fuels from outside the volatile Middle East ?but could simultaneously spur significant increases in global greenhouse gas emissions. Oil sands supporters argue that, given the close relationship between the United States and Canada, the security benefits should trump climate concerns. Yet many advocates of aggressive action on climate change strongly oppose oil sands development. The Canadian Oil Sands rejects both extremes. It assesses oil sands' future prospects along with the resulting impacts of growth on national security and climate change, and recommends how policy can balance both concerns. The lessons learned can be applied not only to the oil sands but to conflicts between energy security and climate change more broadly.