This chapter examines attempts to rehabilitate fanaticism by drawing on its supposed contribution to combatting injustices like slavery. Radical campaigners against slavery called themselves “fanatics,” but in the pejorative tradition fanaticism almost invariably has a negative connotation. On the account given here, the supposed virtues of fanaticism are the actual virtues of something with which it is easily confused: radicalism. The abolitionists were not fanatics but radicals. Nevertheless, philosophical apologists for fanaticism are right to raise questions about the merits of moderation. After a critical examination of some arguments for the view that moderation is the quintessential political virtue it is concluded that, depending on the circumstances, both moderation and radicalism can play the role of corrective virtues.