National security professionals in the United States worry about many threats, real and imagined. In this lecture, I will focus on questions surrounding the threat of a nuclear detonation by an unidentified attacker. How can a country best protect itself against nuclear terrorism? What are the technical limits of attribution after the fact? Deterrence has long been the central pillar of nuclear defence. How would deterrence work against non-state actors? What does deterrence theory say about foes who have no capacity to deliver a nuclear warhead by conventional means? Recently, I worked on these questions with a panel of technical experts. I will present some of our more interesting findings, together with a non-classified (because I do not have security clearance!) survey of the technical issues that underlie each of these questions. If you plan to attend, here is a homework problem, to be thought about and talked about with your friends before you arrive: After the fall of the Soviet Union, there were suddenly many potentially poorly guarded nuclear weapons in the world. Perhaps some are unaccounted for even today. Let us stipulate that missing Soviet weapons presently comprise the greatest nuclear threat to Western democracies. If you were the British Prime Minister, what would you do to protect London from this particular threat?