Siegfried Hecker, former Director of Los Alamos National Laboratory from 1986 through 1997, is a nuclear weapons expert specialising in materials science and one of the world's leading experts on plutonium science. His role was crucial for the maintenance of the U.S. nuclear arsenal without explosive testing. Hecker has visited North Korea several times at the invitation of the government and witnessed first-hand the country's nuclear programme development. In addition, since the end of the Cold War, he has worked on fostering collaboration between Russian and American nuclear scientists. He is now Co-Director of the Center for International Security and Cooperation at Stanford University.Hecker talked to CTBTO Faces in September 2011 while in Vienna to lecture at the CTBT Introductory Course.Disclaimer: The views expressed in this interview do not necessarily reflect the positions of the CTBTO.