This article examines the changing nature of fear in Enlightenment France. While the growing power of the absolutist state reduced many traditional sources of insecurity, fears of state power proliferated during the eighteenth century, prompting leading figures of the French Enlightenment to turn their attention to the problem of political fear: its sources, its effects, and the means for overcoming it. One of the unifying aspects of the Enlightenment was its commitment to reducing the burden of fear in human existence. From that standpoint, however, political fear posed a particular challenge since the objects on which it focused could not be dismissed as purely imaginary. Unlike such traditional religious terrors as hell, purgatory, and the Devil, police agents, police spies, and prisons really existed. And yet political fears too were mediated—and magnified—by collective imaginaries. The fear of imprisonment stands out as a key example of such a phenomenon. Best-selling prison memoirs published in the early 1780s sought to mobilize public opinion against lettres de cachet (administrative arrest warrants) by evoking the horrors of imprisonment, and especially its psychological torments: solitude, tedium, uncertainty about the future, and the looming threat of insanity. In these works, prisoners inhabit a separate self-contained world, helpless before the omnipotent will of their jailers, who rule over them like “oriental despots.” The wide dissemination of terrifying images of the prison contributed to building the public pressure for the abolition of lettres de cachet during the Revolution, but the enormous commercial success of the memoirs suggests that some readers found the depictions of life behind prison walls darkly fascinating as well as terrifying. Much the same could be said of readers’ responses to the exposés of Revolutionary prisons published after Thermidor, the Gothic novels of the late eighteenth and early nineteenth centuries, and the fictional universe of the Marquis de Sade, all of which drew heavily on the carceral imaginary invented under the Old Regime.