Chapter 3 has shown that coordination is a central concern in U.S. counterterrorism policy. An examination of the most important policing components of counterterrorism may reveal the extent and manner in which such coordination efforts have practically been realized since September 11. In this chapter, I first offer an overview of the longstanding involvement in counterterrorism by the Federal Bureau of Investigation (FBI), the lead investigative agency in the U.S. Department of Justice. Next, I examine the major organizational and functional aspects of counterterrorism operations by selected security organizations in the Department of Homeland Security (DHS) structure, specifically U.S. Immigration and Customs Enforcement (ICE) and U.S. Customs and Border Protection (CBP). Also discussed is the counterterrorism role of the Bureau of Diplomatic Security (DS), the main security and law enforcement branch of the U.S. Department of State.