Why do we fear preventive detention and believe that it requires special justification? Citizens in free, democratic societies are accustomed to substantial limitations on liberty that are justified morally and politically by the “common good.” Avoiding serious danger is a greater good than most, but the power of the state to confine dangerous people or to reduce danger by equally oppressive intrusions is considered especially fearsome. The usual, and generally uncontroversial, justifications given for such deprivations of liberty are that the person has culpably committed a criminal offense or that the agent is not responsible for the danger he or she presents. Criminal imprisonment and various forms of civil commitment, for example, which preempt potentially dangerous conduct by incapacitation, are considered reasonable deprivations of liberty in such cases. But in neither case is the state confining on purely preventive grounds a responsible agent who has done no wrong. In contrast, pure preventive detention is an anathema, we believe, because polities devoted to liberty and autonomy have no moral or political warrant to confine or similarly oppress innocent, responsible agents. What could justify such a vast intrusion on the liberty of such agents to pursue their projects?I recognize that the state has an uncontroversial right to quarantine innocent, responsible agents if such agents have communicable diseases and no less intrusive intervention will prevent infection of others. Although many forms of communicable disease can be spread by conduct, the justification of pure quarantine requires no action or potential action. It is a purely public health measure directed toward microorganisms that has the undesirable effect of limiting freedom of action. This article addresses only the preemption of dangerous conduct, which might in some cases include the transmission of disease. See generally Michael Corrado, Punishment, Quarantine, and Preventive Detention, 15 Crim. Just. Ethics 3 (Summer/Fall 1996).