The following dissertation traces the convergence of science, culture, and politics in Portugal’s most important tropical colonies—Goa, India and Bahia, Brazil. It shows how contradictions between an idealized colonial order and the exigencies of settlementpatterned debates over the practice of medicine and, in the process, redefined scientific authority, credibility, and the study of nature. This story began in the Atlantic, where the unexpected virulence of certain fevers along the West African coast challenged European ideas about the causes of disease. To survive, the Portuguese turned to indigenous medical specialists, often women. With colonization in Asia and the Americas, this practice intensified and grew more controversial. Hindu and Muslim physicians in Goa aswell as Amerindian and, later, African-descended healers in Bahia mediated Portuguese access to local flora and its curative uses. The two zones of colonization differed, however, in fundamental ways. The comparative dimension of this project helps clarify these differences so as to explain the emergence of particular kinds of medical interaction within each colonial setting. At the same, I argue that colonial medicine—as it took shape throughout Portugal’s empire—was always built upon a fundamental opposition that was inherent in the project of colonization itself: Portuguese communities became dependent on forms knowledge-making that they simultaneously sought to displace. At issue wasthe authority (and therefore power) that women and non-Christian peoples wielded in the production and verification of truth claims about the natural world. Hence in the colonies, the intertwined disciplines of medicine and natural history confronted tensions andambiguities that distinguished them from their metropolitan counterparts. And this in turn fostered distinct ways of assembling intellectual communities, asserting claims to truth, representing both in print, and defending that work in the face of suspicion andaccusation from colonial governors and Inquisition officials alike.