One of the most devastating attacks a terrorist group could mount would be to detonate an atomic bomb in a city. If exploded in Manhattan during working hours, for example, a bomb with a yield of only 1 kiloton could kill 200,000 people outright and flatten eleven city blocks. In theory, as little as 4 kilograms (9 pounds) of plutonium would be needed to make a bomb. As little as 16 to 20 kilograms of highly enriched uranium would be needed to make an efficient bomb; a crude bomb could be made with 50 to 100 kilograms of uranium. By contrast, the world’s supply of highly enriched uranium is estimated to be 1,600,000 kilograms; the supply of plutonium, 450,000 kilograms. The talk will discuss how to search for special nuclear materials using particle probes such as neutrons, gamma rays, muons and neutrinos and what the detector systems look like that are required to find these threat materials.