This chapter discusses the legal foundations of homeland security in the United States. It covers international and domestic law challenges, the historical context of state-sponsored terrorism, classifying state sponsors and foreign terrorist organizations, international counterterrorism cooperation, and key US legislation related to homeland security including the Patriot Act, the Department of Homeland Security Act, and the Patriot Reauthorization Act. It also examines the sweeping scope of new counterterrorism laws, the watchdog role of Congress, judicial decisions limiting executive authority, and the necessity of increased government authority.