I defend a form of preventive detention through the creation of an offence of ‘being a persistent violent dangerous offender’. This differs from alternative proposals and actual habitual offender laws that impose extra periods of incarceration on offenders after they have completed the sentence for their most recent crime or as a result of a certain number of prior convictions. I, instead, would make ‘being a persistent violent dangerous offender’ an offence itself. Persons to be preventively detained would be tried (...) and convicted of this offence. My approach would then have one significant advantage: provided the elements of being a PVDO could be rendered sufficiently determinate, punishing persons under such an offence would comport with central rule of law values, most importantly legality and fair notice, as well as principles of proportionality in sentencing. (shrink)