Incest is a remarkably frequent theme in medieval literature; it occurs in a wide range of genres, including romances, saints's lives, and exempla. Historically, the Church in the later Middle Ages was very concerned about breaches of the complex laws against incest, which was defined very broadly at the time to cover family relationships outside the nuclear family and also spiritual relationships through baptism. Medieval writers accepted that incestuous desire was a widespread phenomenon among women as well as men. They are surprisingly open about incest, though of course they disapprove of it; in many exemplary stories incest is identified with original sin, but the moral emphasizes the importance of contrition and the availability of grace even to such heinous sinners. This study begins with a brief account of the development of medieval incest laws, and the extent to which they were obeyed. Next comes a survey of classical incest stories and their legacy; many were retold in the Middle Ages, but they were frequently adapted to the purposes of Christian moralizers. In the three chapters that follow, homegrown medieval incest stories are grouped by relationship: mother-son (focusing on the Gregorius legend), father-daughter (focusing on La Manekine and its analogues), and sibling (focusing on the Arthurian legend). The final chapter considers the very common medieval trope of the Virgin Mary as mother, daughter, sister and bride of Christ, the one exception to the incest taboo. In western society today, incest has recently been recognized as a serious social problem, and has also become a frequent theme in both fiction and non-fiction, just as it was in the Middle Ages. This interdisciplinary study is the first broad survey of medieval incest stories in Latin and the vernaculars (mainly French, English and German). It situates the incest theme in both literary and cultural contexts, and offers many thought-provoking comparisons and contrasts to our own society in terms of gender relations, the power of patriarchy, the role of religious institutions in regulating morality, and the relationship between life and literature.