This dissertation explains how the body provided proof of holiness in early modern Catholicism. Tradition had long asserted that the bodies of especially saintly or sinful individuals should reveal signs of their elect status. In the early modern period, Church officials sought to make such vague ideas exact: they required medical proof of bodily holiness. This requirement arose out of developments in both medicine and Catholicism. After the Reformation, many Christians doubted how or even if they should venerate the saints. In response, the Catholic Church centralized and formalized canonization procedures. As part of this reform, canonization officials began to require that medical practitioners examine the cadavers of holy individuals. Medical and anatomical knowledge had progressed during this period to the point where dissection was thought to yield unique information about the lifestyle and habits of the deceased. This information could also include signs of a person's spiritual state. The body, interpreted through expert eyes, could now reveal a person's true nature. Together, medical practitioners and canon lawyers in the city of Rome created standards through which to interpret bodily signs of holiness. The methods of proof employed in these examinations presaged many techniques of knowledge production that would become a part of experimental practice in the early modern world. This dissertation therefore demonstrates that instead of being opposed, science and religion worked together to define the boundaries of the natural in human bodies.