The Amazon Rainforest became an urban region during the twentieth century. In Brazil and Peru, more than seventy percent of the almost 26 million inhabitants of Amazonia live in cities. My dissertation examines the environmental history of Manaus (Brazil) and Iquitos (Peru), the two largest and most important cities of Amazonia during the twentieth century. Drawing from local, regional and national archives, as well as from select interviews, I explore the relationships between the environmental conditions of the rainforest and the social and spatial inequalities that characterize Latin American cities. I analyze critical junctures marked by the confluence of four key urban environmental issues: 1) the history of land ownership, 2) the history of water management, 3) the history of public health, and 4) the history of crime. Massive urbanization radically transformed Amazonia by reshaping everyday life, local social structures, and human interactions with the environment. In the large popular neighborhoods at the edges of Iquitos and Manaus, urban informality created distinct social landscapes that challenged prevailing dichotomies between city and rainforest, between urban and rural, and between built environments and nature. Meanwhile, developmental policies and contingent political factors took Manaus and Iquitos to different paths. Ultimately, convergences of popular agency, environmental conditions, and the forces of state formation and capital shaped the making of urban spaces in Amazonia. Neither cultural nor environmental determinisms explain the divergences between the two jungle cities. Both comparative and transnational, the history of the urbanization of the planet's largest tropical rainforest speaks to larger debates about of the complex, changing, and multidirectional relationships between humans and their environments.