Conversations host Harry Kreisler welcomes Siegfried S. Hecker, former Director of the Los Alamos National Laboratory, for a discussion of scientists, the national laboratories, and the threat posed by nuclear weapons. Hecker traces his career in material sciences, describes the evolution of his intellectual focus, and recalls his leadership of Los Alamos. He then traces the changes in the international security environment in the aftermath of the collapse of the Soviet Union discussing the response of the U.S. and the weapons laboratories to the momentous events that created a qualitatively different set of security challenges. Hecker then analyzes the threats posed by terrorist organizations, the dangers of nuclear proliferation, and the challenges for U.S. policy in assessing the motivation and capabilities of Pakistan, North Korea, and Iran. He emphasizes the importance of understanding the political and technical dimensions of the international security landscape.