This paper offers a brief overview of the principal forms and strategies of authorization in early modern medicine and of the problems and challenges to which they gave rise. The first part of the paper looks at the various ways in which physicians—and, in particular, the authors of medical treatises and textbooks—sought to lend authority to their findings and explanatory models within the learned medical debate. The traditional triad of ratio, experientia, and auctoritates remained central to any claim to authority. There was a substantial shift over time, however, from validating truth claims by quoting authorities to empirical observation, not only in anatomy and botany but also and most of all in practical, clinical medicine, as evidenced by the rise of collections of case histories and medical observationes. In practical medicine, the problem of authorizing truth claims was particularly urgent, because the physicians frequently experienced that their treatment with blood letting, laxatives, emetics, fontanels, and complex mixtures of medicinal plants often failed. In their dealings with patients and relatives the physicians underscored their learning by offering detailed theoretical explanations of the disease processes taking place inside the body. In the long run, these strategies were remarkably successful and upper-class patients came to prefer the services of doctors of medicine.