We face two global threats: the climate crisis and a crisis of democracy. Located at the crux of these crises, sustainable cities build on the foundations and resources of democracy to make our increasingly urban world more resilient and just. Sustainable Cities in American Democracy focuses on this effort as it emerged and developed over the past decades in the institutional field of sustainable cities—a vital response to environmental degradation and climate change that is shaped by civic and democratic action. Carmen Sirianni shows how various kinds of civic associations and grassroots mobilizing figure in this story, especially as they began to explicitly link conservation to the future of our democracy and then develop sustainable cities as a democratic project. These organizations are national, local, or multitiered, from the League of Women Voters to the Natural Resources Defense Council to bicycle and watershed associations. Some challenge city government agencies contentiously, while others seek collaboration; many do both at some point. Sirianni uses a range of analytic approaches—from scholarly disciplines, policy design, urban governance, social movements, democratic theory, public administration, and planning—to understand how such diverse civic and professional associations have come to be both an ecology of organizations and a systemic and coherent project. The institutional field of sustainable cities has emerged with some core democratic norms and civic practices but also with many tensions and trade-offs that must be crafted and revised strategically in the face of new opportunities and persistent shortfalls. Sirianni’s account draws ambitious yet pragmatic and hopeful lessons for a “Civic Green New Deal”—a policy design for building sustainable and resilient cities on much more robust foundations in the decades ahead while also addressing democratic deficits in our polarized political culture.