Fissile material -- highly-enriched uranium (HEU) or weapon-grade plutonium -- is the critical ingredient in building a nuclear weapon. Most experts agree that terrorists are not able to produce fissile material, but a reasonably sophisticated terrorist organization could make a crude nuclear weapon, or improvised nuclear device (IND), if it stole or acquired a sufficient quantity and quality of such material. Therefore, combating smuggling of weapons-usable nuclear materials is vital to preventing terrorists from acquiring nuclear weapons.