This article analyzes the grounds, procedures, and conditions required by International Human Rights Law for preventive detention of suspected terrorists as threats to security. Such detention is generally permitted, provided it is based on grounds and procedures previously established by law; is not arbitrary, discriminatory, or disproportionate; is publicly registered and subject to fair and effective judicial review; and the detainee is not mistreated and is compensated for any unlawful detention. In Europe, however, preventive detention for security purposes is generally not permitted. If allowed at all, it is permitted only when a State in time of national emergency formally derogates from the right to liberty under the European Convention on Human Rights. The article concludes that if preventive detention for security purposes is to be allowed at all, its use must be kept to an absolute minimum, and the European model should be followed, allowing detention only by formal derogation during national emergency, and then only to the extent and for the time strictly required.