In the late twelfth century, northern European Jewish mystics engaged in a sustained, unprecedented effort to explore the theological meaning of werewolves. This article seeks to anchor this surprising preoccupation in contemporary European religious culture, arguing that medieval Jews and Christians found werewolves “good to think with” in exploring the spiritual status of the (mutable, unstable) human body. Discourses of monstrosity were used as polemical ammunition in Jewish-Christian debates, but monstrous creatures were simultaneously held to be theologically resonant by both communities—a fact that sheds light upon the broader intellectual and cultural setting in which they were joint participants.