The nation’s air, land, and marine transportation systems are designed for accessibility and efficiency, two characteristics that make them highly vulnerable to a terrorist attacks. While hardening the transportation sector from terrorist attacks is difficult, reasonable measures can be taken to deter terrorists. The focus of this issue brief is how best to construct and finance a system of deterrence, protection, and response that effectively reduces the possibility and consequences of another terrorist attack without unduly interfering with travel, commerce, and civil liberties.