The controversy about vaccines and autism presents an opportunity to explore how science is constructed in public debates about health and medicine. Rhetors who argue against a connection between vaccines and autism insist that their opponents are irrational, while rhetors arguing for a link insist that their fears are rational indeed. This analysis poses an alternative way of understanding the vaccines-autism controversy, suggesting that it is partly fueled by differing perceptions of the boundary between science and non-science. Using the concept of boundary work as a lens, this article uses generative rhetorical criticism to examine artifacts within the controversy and explores rhetorical constructions of scientific evidence, the forum of scientific discourse, scientific expertise, and the scientific capability. The findings suggest that rhetors’ awareness of disciplinary boundaries is just as important in the construction and reception of their arguments as their knowledge of scientific facts and principles.