Five nations openly possess nuclear weapons and nearly a dozen more have been reported or rumored to have nuclear capability or to be working toward it.
The five that have nuclear weapons are the United States, Soviet Union, Britain, France and China.
No other country has acknowledged possessing nuclear weapons publicly, and most of those thought to be developing them have denied any interest in building a bomb and have said their nuclear programs are for peaceful uses.
India, Israel and South Africa are generally considered in the ambiguous stage -- possibly possessing nuclear weapons or the technology and materials to make them, and Pakistan may have joined this group, according to a report last fall by the Carnegie Endowment for International Peace.
India detonated a nuclear "device" in 1974 and has since expanded its ability to produce plutonium. Israel reportedly has built 10 to 20 small bombs, and South Africa, according to Carnegie, may have enough highly enriched uranium to make 25 small bombs and actually may have built some.
Pakistan said a year ago that it has no plans to build a nuclear bomb, but it is thought to have the capability and the production facilities for weapons-grade uranium.
At a less immediate level, Argentina has the most advanced nuclear program in Latin America but is thought to be several years from weapons capability, and Brazil said a year ago it would be able to produce a nuclear weapon by the 1990s.
Iraq's efforts, according to the Carnegie report, have been largely "dormant" since Israel bombed its reactor in June 1981. Libya reportedly has tried to buy nuclear technology, and the State Department said last year that it had no information to substantiate rumors that Iran had the equipment to make a nuclear bomb.
South Korea and Taiwan once were considered potential nuclear powers, but there has been no recent indication that they are pressing forward.