Before the end of the thirteenth century, theologians had little interest in demons, but with Thomas Aquinas and his formidable “Treatise on Evil” in 1272, everything changed. In Satan the Heretic, Alain Boureau trains his skeptical eye not on Satan or Satanism, but on the birth of demonology and the sudden belief in the power of demons who inhabited Satan’s Court, setting out to understand not why people believed in demons, but why theologians—especially Pope John XXII—became so interested in the subject. Depicting this new demonology, Satan the Heretic considers the period between the mid-thirteenth and mid-fourteenth centuries when demons, in the eyes of Church authorities, suddenly burst forth, more real and more terrifying than ever before in the history of Christianity. Boureau argues that the rise in this obsession with demons occurs at the crossroads of the rise of sovereignties and of the individual, a rise that, tellingly, also coincides with the emergence of the modern legal system in the European West. Teeming with original insights and lively anecdotes, Satan the Heretic is a significant contribution to the history of Christian demonology from one of the most original minds in the field of medieval studies today.