Since 1930, Belgium has a law for the protection of society aimed at guarding society against the dangerously insane. That law responded to one of the prevailing issues in late 19th century Europe: the question of “abnormal individuals”, a category that included insane offenders perceived as dangerous. The 1930 Social Defense Act prescribed a special regime for insane offenders and immediately demonstrated deep-seated ambivalence. We propose to analyze the origin of this law, its content and its evolutions over time by adopting an approach taking into account long duration. This approach allows us to highlight continuities and discontinuities in the treatment of criminal insanity and dangerousness over the last century. We also rely on several recent empirical inquiries dedicated to the regime of insane offenders oscillating between care and safety, the assessment of dangerousness and the role held by expertise, insane offender’s trajectory and their release increasingly characterized by risk management approach and techniques.