From the time of its composition (c. 1280) for Philip the Fair of France until the early sixteenth century, Giles of Rome's mirror of princes, the De regimine principum, was read by both lay and clerical readers in the original Latin and in several vernacular translations, and served as model or source for several works of princely advice. This study uses an interdisciplinary approach toward the surviving manuscript copies, as well as documentary and literary evidence, to show how people of the later Middle Ages read Giles' text and appropriated it for their own particular purposes.