This is the first scholarly study of inquisition into heresy in medieval England. Heresy was the most feared crime in the medieval moral universe. It was seen as a social disease capable of poisoning the body politic and shattering the unity of the church. Ian Forrest argues that in order to understand who the heretics were, we first need to understand how they were defined in their own day. Successful heresy-detection relied on balancing suspicion against legal standards of evidence, and weighing the safety of the public against the rights of the suspect.