In the second half of the nineteenth century, American cities began to go dark. Hulking new buildings overspread blocks, pollution obscured the skies, and glass and smog screened out the health-giving rays of the sun. Doctors fed fears about these new conditions with claims about a rising tide of the “diseases of darkness,” especially rickets and tuberculosis. This book tracks the obsession with sunlight from those bleak days into the twentieth century. Before long, social reformers, medical professionals, scientists, and a growing nudist movement proffered remedies for America's new dark age. Architects, city planners, and politicians made access to sunlight central to public housing and public health, and entrepreneurs, dairymen, and tourism boosters transformed the pursuit of sunlight and its effects into a commodity. Within this historical context, the book examines questions about the commodification of health and nature and makes a contribution to the histories of cities, consumerism, the environment, and medicine.