Apart from the five permanent members of the UN security council (the US, Russia, China, France, and Britain), which are all nuclear powers, around 25 other countries have sought to obtain nuclear weapons, say international analysts.
Israel, India, Pakistan, and South Africa are known to have succeeded, though South Africa voluntarily relinquished its bomb in the 1990s, a unique event.
North Korea is feared to be building a bomb, Saddam Hussein's Iraq tried and failed, and Iran is said to be creating the capacity.
Japan has the fissile material and the know-how to develop one quickly. Other countries occasionally rumored to have nuclear ambitions include Brazil, Argentina, Libya, and Algeria.
It has never officially declared itself a nuclear power but is by some distance the mightiest nuclear power outside the Big Five, with the first prime minister, David Ben-Gurion, inaugurating the project in the mid-1950s in great secrecy. It had a rudimentary device by the late 60s. Mordechai Vanunu, a nuclear technician, famously blew the whistle on the programme in 1986 and was imprisoned.
France is believed to have supplied Israel secretly with a nuclear reactor and equipment for extracting weapons-grade plutonium from spent nuclear fuel in the 1950s.
Israel is estimated to have stockpiles of more than half a tonne of plutonium and an unknown quantity of weapons-grade high-enriched uranium. The plutonium stockpile is more than the combined total held by the other non-Big Five nuclear powers. Experts estimate that Israel has around 200 nuclear devices.
For decades India pursued the bomb using a plutonium extraction plant and two heavy water reactors which produce plutonium. The Indian effort was based at the Bhabha atomic research centre outside Bombay. It went public in 1974 when a plutonium-core fission bomb was tested in the Rajasthan desert.
Analysts estimate that the Indian nuclear arsenal has up to 70 devices.
The Indian successes triggered a nuclear arms race on the sub- continent. The father of the Pakistani bomb, Abdul Qader Khan, worked in the Netherlands for what became the Anglo-Dutch-German Urenco company specializing in uranium enrichment.
He is said to have stolen centrifuge designs for uranium enrichment and inaugurated the crash Pakistani programme. The same designs have been used at the Iranian facility at Natanz, feeding speculation that the Iranians obtained equipment and help from Islamabad.
The Pakistanis built the Kahuta enrichment plant using the Urenco designs to produce weapons-grade uranium, and are believed to have obtained weapons designs from China. Pakistan announced it had the bomb in 1998.
Pakistan is the only country known with certainty to have a bomb developed from a programme started after the nuclear non-proliferation treaty came into force in 1970. It is believed to have at least 700kg of weapons-grade uranium and a small quantity of plutonium stockpiled and to have up to 15 nuclear devices.
The current crisis over North Korea's nuclear ambitions came to a head last year when Pyongyang cut off relations with the UN watchdog, the Vienna-based International Atomic Energy Agency, and said it would not observe the non-proliferation treaty.
The jury is still out on whether North Korea has the bomb. It said last month it could test a nuclear device when it wanted. Washington believes it may have two.
Multilateral efforts are under way to defuse the crisis, which is having the knock-on effect in Japan of encouraging the only country to have been hit by an atomic bomb to build its own. Japan has a large civil nuclear power sector, ample stockpiles of plutonium, and the technological and scientific resources to obtain a nuclear bomb swiftly.
A dripfeed of revelations from UN inspectors over recent months is hardening suspicions that Tehran is bent on acquiring nuclear weapons, or creating the wherewithal. It was last week presented with an ultimatum by the UN: reveal all by the end of next month and allow unrestricted inspections.
Recent discoveries and question marks include the tracing of two different types of weapons-grade uranium at an underground uranium enrichment centre being built in central Iran, the disclosure that the programme goes back to 1985 and not 1997 as previously stated, evidence of uranium metal conversion testing -- needed for weapons but not for power stations -- and substantial rebuilding at a suspect Tehran facility, apparently to frustrate UN inspections and environmental sampling for radioactive materials.