Historians of science have often looked to the authentication of miracles at canonization trials as a way to investigate the ways in which religious and scientific understandings of the natural and the miraculous came together and, sometimes, into conflict. Most historians of science who have forayed into the world of miracles have, understandably, stopped at the moment of a saint's canonization. Examining the treatment of a saint's miracles both before and after the canonization process, however, yields a different picture. Drawing upon materials from the 1455 canonization and subsequent cult of the Dominican Vincent Ferrer (1350–1419), this essay reveals, first, that papal approval marked only one of several ways in which miracles received publicly-accepted “authentication,” and second, that, after the moment of canonization, the idea of carefully authenticated miracles became irrelevant not simply for the great masses of the faithful, but also for the ecclesiastical hierarchy, who adopted an ever shifting rhetoric of authenticity as authors used tales of the saint's “authentic” miracles to drive home their own various polemical points.