This chapter describes the role of Roman law—whose influence has been largely underestimated in recent scholarship—in the intellectual history and development of international law. To that end, the chapter offers a general survey of the historical interactions between Roman law and international law, drawing from general insights into the intellectual history of law in Europe that have remained remarkably absent in the grand narrative of the history of international law. The focus is on the periods in which these interactions were most pronounced. Next to Roman Antiquity, these are the Late Middle Ages (eleventh to fifteenth centuries) and the Early Modern Age (sixteenth to eighteenth centuries).