Delegates from more than 180 countries are gathering in New York to review the Nuclear Non-Proliferation Treaty, 35 years after it came into force. The review takes place every five years to assess how the treaty can be strengthened and to check how well signatories are meeting their obligations.
BBC News Website looks at the key issues.
What is the Nuclear Non-Proliferation treaty?
The NPT, which has 187 signatories, was created to prevent new nuclear states emerging, to promote co-operation in the peaceful use of nuclear energy and to work towards nuclear disarmament.
When it came into force in 1970, there were five declared nuclear states - the US, The Soviet Union (now Russia), China, Britain and France. These states are bound not to transfer nuclear weapons or to help non-nuclear states to obtain them.
Non-nuclear signatories agree not to seek to develop or acquire such weapons. In return, they are given an undertaking that they will be helped to develop nuclear energy for peaceful purposes.
Has it been successful?
Over the years, the NPT security framework has led several states to abandon their nuclear weapons ambitions. It has also made it far more difficult for other states to acquire the material and technology needed to build such weapons.
South Africa and Ukraine gave up their nuclear weapons, and the whole of South America remains nuclear-free.
One of the most recent successes in non-proliferation came in 2003, when Libya renounced its secret efforts to acquire nuclear capability.
However, there are three states - India, Israel and Pakistan - which are known to possess nuclear weapons but have never joined the treaty.
What do critics of the treaty want to change?
The US and its allies are concerned about the nuclear programmes in Iran and North Korea, and have tabled amendments to the treaty which would make it harder for other countries to go down the same path.
Both developed nuclear capability as NPT members. The treaty allows countries to make nuclear fuel for peaceful purposes, through enriching uranium or separating plutonium. But these technologies also have the potential to be used to make nuclear weapons.
In 2003, North Korea withdrew from the treaty and claimed it was building nuclear weapons. Under international law, NPT countries were institutionally powerless to respond.
Iran is pursuing an uranium enrichment programme, insisting that its intentions are strictly civilian. Washington, however, suspects Iran wants to build a nuclear bomb, and wants Tehran to halt all nuclear activities.
The Bush administration has proposed a number of treaty amendments, the most important of which would stop the spread of enrichment technology.
It also reportedly favours a German-French proposal that would require countries withdrawing from the NPT to give back benefits derived from the treaty, such as technology associated with peaceful nuclear energy production.
Are the main nuclear powers complying with the treaty?
Many nations without nuclear arms see the continued existence of huge stockpiles among the five declared nuclear powers as the main problem with the treaty.
Article VI of the treaty says: "Each of the Parties to the Treaty undertakes to pursue negotiations in good faith on effective measures relating to cessation of the nuclear arms race at an early date and to nuclear disarmament, and on a Treaty on general and complete disarmament under strict and effective international control."
The US, in particular, has caused concern with its plans to develop and test new weapons, including anti-ballistic missiles, the earth-penetrating "bunker buster" and perhaps some new "small" bombs.
Critics say these plans clash with 13 steps agreed at the last review in 2000, which includes calls for "a diminishing role" for nuclear weapons in security policies.
What about nuclear countries outside the NPT?
Non-nuclear and strongly anti-nuclear countries like New Zealand regard the fact that Israel, India and Pakistan are subject to none of the treaty's restrictions as a major problem.
Iran also says that this is unfair and that Israel should be forced to give up its nuclear weapons. But Israel says that it is in special peril.
India and Pakistan argue that if the US and others have nuclear weapons for defence, and proclaim the value of nuclear deterrence, they have no right to deny others the same benefits.
However, some experts say that the failure to bring these countries into the NPT may tempt others to join them outside.