This article analyzes the grounds, procedures and conditions required by International Human Rights Law and International Humanitarian Law for pretrial detention of suspected terrorists for purposes of criminal law enforcement, and for their preventive detention for security and intelligence purposes. Recognizing the difficulties in securing sufficient admissible evidence to prosecute terrorists within the tight time limits imposed by international law, the Article nonetheless suggests that indefinite detention, solely or primarily for purposes of intelligence interrogation, is probably not lawful under U.S. or international law. Preventive detention for security purposes, on the other hand, is generally permitted by international law, 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, even with these safeguards, preventive detention for security purposes is generally not permitted, unless a State in time of national emergency formally derogates from its obligation to respect the right to liberty under the European Convention on Human Rights. The Article concludes that if preventive detention of suspected terrorists for security purposes is to be allowed at all, its inherent danger to liberty must be appreciated, its use kept to an absolute minimum, and the European model should be followed, that is, such detention should be permitted only by formal derogation in time of national emergency, and then only to the extent and for the time strictly required.