By the late 19th century, American cities like Chicago and New York were marvels of the industrializing world. The shock urbanization of the previous quarter century, however, brought on a host of environmental problems. Skies were acrid with coal smoke, and streams ran fetid with raw sewage. Disease outbreaks were as common as parks and green space was rare. In response to these hazards, particular groups of urban residents responded to them with a series of activist movements to reform public and private policies and practices, from the 1890s until the end of the 20th century. Those environmental burdens were never felt equally, with the working class, poor, immigrants, and minorities bearing an overwhelming share of the city’s toxic load. By the 1930s, many of the Progressive era reform efforts were finally bearing fruit. Air pollution was regulated, access to clean water improved, and even America’s smallest cities built robust networks of urban parks. But despite this invigoration of the public sphere, after World War II, for many the solution to the challenges of a dense modern city was a private choice: suburbanization. Rather than continue to work to reform and reimagine the city, they chose to leave it, retreating to the verdant (and pollution free) greenfields at the city’s edge. These moves, encouraged and subsidized by local and federal policies, provided healthier environments for the mostly white, middle-class suburbanites, but created a new set of environmental problems for the poor, working-class, and minority residents they left behind. Drained of resources and capital, cities struggled to maintain aging infrastructure and regulate remaining industry and then exacerbated problems with destructive urban renewal and highway construction projects. These remaining urban residents responded with a dynamic series of activist movements that emerged out of the social and community activism of the 1960s and presaged the contemporary environmental justice movement.