In some jurisdictions, the institutions of criminal justice may subject individuals who have committed crimes to preventive detention. By this, I mean detention of criminal offenders (i) who have already been punished to (or beyond) the point that no further punishment can be justified on general deterrent, retributive, restitutory, communicative or other backwardlooking grounds, (ii) for preventive purposes—that is, for the purposes of preventing the detained individual from engaging in further criminal or otherwise socially costly conduct. Preventive detention, thus understood, (...) shares many features with the quarantine measures sometimes employed in the context of infectious disease control. Both interventions involve imposing (usually severe) constraints on freedom of movement and association. Both interventions are standardly undeserved: in quarantine, the detained individual deserves no detention (or so I will, for the moment, assume), and in preventive detention, the individual has already endured any detention that can be justified by reference to desert. Both interventions are, in contrast to civil commitment under mental health legislation, normally imposed on more-or-less fully autonomous individuals. And both interventions are intended to reduce the risk that the constrained individual poses to the public. Yet despite these similarities, preventive detention and quarantine have received rather different moral report cards. (shrink)