This dissertation seeks to understand the idea of a boundary between humans and non-human creatures in the early modern era. The idea of a boundary between people and non-people, while implicit among most sixteenth-century theologians, is still an important feature of early modern history. However, the boundary, while rhetorically very important and static, did not match with the reality of the boundary in theology and culture as fluid. Theologians argued at length that humanity, being made in the “image of God,” retained a fundamental difference from animals and other nonhuman creatures, in which that boundary could not be crossed. However, they also allowed for animals to possess positive traits and even moral and legal culpability. They also accepted creatures that challenged the boundary, whether monsters (including exotic creatures and misbirths) or humans who were not thought to possess all of the constituent characteristics of the “image of God,” such as those with mental or cognitive deficiencies. Thus, they struggled to reconcile the experiential reality of a fluid boundary with the theological conviction of an anthropocentric hierarchy of creation. This dissertation will address the inherent tension between these two views and assess the ways in which theological and cultural figures helped to resolve the tension. Using early modern commentaries on Genesis, we will first examine the rhetorical insistence on a firm boundary articulated by figures both mainstream and heterodox. Then, we will examine the popular perception of a fluid boundary, in which nonhuman creatures could be addressed and understood morally in bestiaries, saints’ lives, and trial records. Finally, we will examine how proto-scientific thinkers of the sixteenth century, like Conrad Gessner, Andreas Vesalius, Johann Weyer, and Ambroise Paré, actively challenged existing authorities and helped to resolve the tension to a state in which humans and nonhuman creatures were different, yet both existed within the broader sphere of nature. By the end of the sixteenth century, violations of the boundary between people and non-people come to be rejected more for their natural than theological implications.