The strategic and operational challenges facing these units are unlike any faced by domestic law enforcement agencies in the past. These units were formed in the belief that there are or will be terrorist cells operating in the United States to devise and execute attacks on American citizens for no other reason than to kill as many as possible. The efforts of these units thus qualify as "pioneering" efforts, because they require an unprecedented sharing of the collection and dissemination of intelligence across agencies and jurisdictions. Investigative methods must become more proactive than reactive because of the disastrous consequences of a launched terrorist attack. Intelligence sources are being cultivated in the private sector, since this will increase the number and placement of individuals and groups who may have useful information about suspicious persons and activities. Domestic law enforcement counterterrorist units have to expand their knowledge of foreign cultures and events related to international terrorism. Counterterrorist units must also be in regular communication with law enforcement units that deal with criminal enterprises in which terrorists are likely to be involved, such as drug trafficking, extortion, money laundering, and prostitution. Counterterrorist units also focus on target hardening, as they work with private- sector and public sector enterprises with facilities that may be terrorist targets. All of these new counterterrorism tactics are requiring law enforcement agencies to create new organizational structures that can accommodate the pioneering innovations in intelligence and proactive policing. This article also presents some "success stories" and identifies remaining challenges.