"Boundaries of Nature" investigates the creation of the Iguazú National Park in Argentina (1934) and the Iguaçu National Park in Brazil (1939), analyzing the geopolitical reasoning and the spatial practices behind the establishment of these two protected areas. Located at the Argentine-Brazilian border around the famous binational Iguazu Falls, the two national parks were initially envisioned as tools for the nationalization of the border. In Argentina, park proponents innovated by engaging the country's national park service in the promotion of settler colonization in national park lands. The Argentine national park agency traced street grids, parceled and sold lots, and implemented urban infrastructure inside park boundaries. Brazilian officials in turn, partially inspired by the Argentine example across the border, engaged in more modest efforts to use national park policy in the development of their side of the border. Brazil's weak control of public land, however, resulted in thousands of contested settlements inside its own border national park. In the 1960s and 1970s, the consolidation of an international paradigm of national parks as spaces devoid of dwellers and the strengthening of state tools to manage land and people led the military regimes in the two countries to engage in conflictive processes of settler eviction. This study employs space as a crucial dimension in the reconstruction of the history of Iguazú and Iguaçu, showing the spatial practices that helped to define these protected areas: mapping, demarcation, zoning, parceling, patrolling, eviction. It also demonstrates how settlers, poachers, and heart-of-palm harvesters contested the enforcement of national park rules in the space of the parks. The result was the establishment of landscapes of protected nature that were socially and politically constructed through spatial processes.