This book explores the Federal Bureau of Investigation (FBI) as the primary investigative agency of the federal government, which has the authority and responsibility to investigate all violations of federal law that are not exclusively assigned to another federal agency. The FBI is further vested by law and by Presidential directives with the primary role in carrying out investigations within the United States of threats to the national security. This includes the lead domestic role in investigating international terrorist threats to the United States, and in conducting counterintelligence activities to meet foreign entities' espionage and intelligence efforts directed against the United States. As discussed in this book, the FBI is also vested with important functions in collecting foreign intelligence as a member agency of the U.S. Intelligence Community. The FBI accordingly plays crucial roles in the enforcement of federal law and the proper administration of justice in the United States, in the protection of the national security, and in obtaining information needed by the United States for the conduct of its foreign affairs. These roles reflect the wide range of the FBI's current responsibilities and obligations, which require the FBI to be both an agency that effectively detects, investigates, and prevents crimes, and an agency that effectively protects the national security and collects intelligence.