||The World Trade Center attacks on September 11, 2001 demonstrated the threat of terrorism in the twenty-first century-and made the prevention of nuclear terrorism all the more pressing. Despite the newfound attention to nuclear terrorism, the possibility of nuclear attacks, and, later, nuclear terrorism, had been a concern of the U.S. government since the early decades of the Cold War. Accordingly, the National Intelligence Estimate reports increasingly addressed the possibility of a covert Soviet nuclear attack and attempts of terrorist groups to acquire and deploy a nuclear weapon. The measures taken to prevent either of these possibilities from becoming a reality took a variety of forms. Preventing the smuggling of nuclear weapons or materials into the U.S. became an overarching mission through more stringent port and airport security and the inception and application of foreign assistance programs, including the Antiterrorism Assistance Program (ATA) and the Cooperative Threat Reduction (CTR) program. The security of nuclear weapons and materials within the United States itself also received repeated scrutiny, resulting in numerous revisions of Atomic Energy Commission (AEC) guidelines regulating the transportation of nuclear materials. Prevention, while an important aspect of protection, was only the first step. Following an attempted extortion involving a nuclear bomb in 1974, the Nuclear Emergency Search Team (NEST) was created, meant to be a group of specialized scientists who could locate and either safely detonate or disarm a nuclear weapon. This thesis examines how the U.S. government developed these responses-preventive and reactive-to the growing threats of nuclear terrorism.