Centuries before being included in Hugo Grotius’s De iure belli ac pacis and De iure praedae, the subject of reprisal was already being discussed in medieval literature. The aim of this paper is to examine the medieval and early modern practice and theory of reprisal as it developed before and during Grotius’s lifetime. Its first part investigates a number of important foundational elements, such as the issues of definition and terminology, and the common characteristics of a reprisal case. In the second half, the author explores why reprisals were deemed to be a ‘perversus mos’ or ‘bad custom’ and how continued reliance on this practice was nonetheless justified by inserting it into the medieval just war doctrine. The paper does not provide a systematic study of Grotius’s own engagement with medieval reprisal sources. Rather, it should be read in conjunction with another publication in this same volume, ‘Grotius on Reprisal’ by Randall Lesaffer.