[영문]The explosion in the 21st century of terrorist activities by Islamic radicals in the United States, Europe and Asia requires reforming the institutions for domestic counterterrorism (CT) and new international relations among individual national CT organizations. This paper discusses the institutional reforms for CT in the United States, focusing particularly on the changes in the FBI. These changes are compared with the way that the British CT activities of the MI5 and MI6 have evolved in response to terrorism in Britain. The paper also discusses the reasons why there is strong cooperation among the CT activities of all the major governments and with the United States in particular, even when those governments do not agree about military cooperation or about the use of economic sanctions.