This thesis investigates the widely held medieval belief that magic could cause sexual dysfunction. It focuses on the period 1150-1450, but also surveys the evidence from the ancient world and the early Middle Ages (up to c.1100). Most of the evidence comes from academic texts that were used for teaching in medieval universities: confessors' manuals; commentaries on canon law; commentaries on the Sentences of Peter Lombard; and medical compendia. There are also magical texts, trial records from the late Middle Ages, and a body of anecdotes, in which people claimed that they had witnessed cases of impotence magic. The thesis analyses how far the academic sources reflect magical practices that existed outside the universities. It also assesses the impact of wider social and intellectual changes on these sources. These changes were: firstly, the growing interest of the church in regulating the spiritual lives of the laity from the late twelfth century onwards; secondly, the translation of magical texts into Latin; and thirdly, the development of the idea that there existed a sect of devil-worshipping witches. My research indicates that learned discussions of magic and impotence began in response to popular concerns. The first authors to discuss the problem in detail did not draw significantly on surviving earlier sources, and claimed instead to have encountered cases of impotence magic. This pattern of learned writer drawing on popular magical practices continued in the later Middle Ages, especially in periods when the authorities tried to regulate what the rest of the population was doing. In canon law and theology, the key period was the thirteenth century, coinciding with the church's drive to reform lay religious beliefs and practices. In medicine, the key period was the early fifteenth century, a time of increased concern about magic in general, which saw the first large-scale witch trials.