When can a person be detained by the state solely for the purpose of preventing future harm? It is widely accepted that an actor who is unable to avoid breaking the law because of a mental disorder may not be punished but may be detained for as long as he remains dangerous. But what about those who are not legally insane and who may be held responsible for their behavior? Is it ever permissible to detain them to prevent future harm? Once upon a time the negative answer to this question was also widely accepted: no one who is sane and responsible for his behavior may be detained solely on the ground that he was dangerous and might commit crimes in the future. He might be punished for his behavior, but he might not be detained independently of punishment. However, over the last thirty years the answer to the question has changed. It is now possible (1) to detain before trial solely on the basis of the possibility that the accused will commit the sort of crime he is accused of (but not yet convicted of); (2) in many jurisdictions to detain indefinitely after trial, conviction, and completion of the penal sentence sex offenders and those found guilty but mentally ill (though not legally insane); and (3) to detain indefinitely without trial and conviction those suspected of being terrorists or supporting terrorist activity. This book traces the development in Supreme Court cases and in national legislation of these various grounds of preventive detention, a course of development that the author believes is contrary to what were once considered fundamental principles of American law.