Since Saddam Hussein was toppled in April 2003 evidence has emerged of corruption within the oil-for-food (OFF) programme administered by the UN.
The BBC News website examines the key questions.
Q: What was the oil-for-food programme?
It was a $60bn (£32bn) scheme which was supposed to allow Iraq to buy food, medicine and other humanitarian supplies with the proceeds of regulated oil sales, without breaking the sanctions imposed on it after its 1990 invasion of Kuwait.
The programme - set up in 1996 - aimed to relieve the suffering of ordinary Iraqis under the sanctions.
It was formally ended in 2003 after the US-led invasion of Iraq and the overthrow of Saddam Hussein.
Q: What is the scandal?
The scandal emerged in early 2004, after an Iraqi newspaper published a list of about 270 people including UN officials, politicians and companies it alleged may have profited from the illicit sale of Iraqi oil during the OFF programme.
Later, US Senate investigators concluded that the Iraqi regime under Saddam Hussein made about $13.6bn from selling oil to neighbour states keen to breach the sanctions.
Billions more were allegedly earned through kickbacks and illegal surcharges on services and goods provided by companies contracted under the OFF programme, the Senate investigators found.
The UN has come under fire as the organisation responsible for administering the OFF programme.
Q: What is being done about the scandal?
A number of investigations have been launched.
The UN set up a three-member independent inquiry, chaired by former US Federal Reserve chairman Paul A Volcker, and also including Justice Richard Goldstone of South Africa and Professor Mark Pieth of Switzerland.
The panel's findings, condemning mis-administration and corruption in the programme, were published on 7 September.
Separate inquiries are under way in the US Senate and by the Iraqi interim government.
The US justice department has also set up a criminal investigation. So far, one man has pleaded guilty and three others have been indicted.
Q: Who has been implicated?
Politicians and UN officials from dozens of countries are suspected of receiving bribes to lobby against sanctions on Iraq's behalf.
International firms - mainly from the Middle East and Russia, but including some large US oil companies - are also being investigated.
A key figure is the former head of the oil-for-food programme, Benon Sevan. In February, an interim report by Mr Volcker's panel into the scandal said he had tried to allocate oil sales from Iraq to one company.
Following that report, both Mr Sevan and another official, Joseph Stephanides, were suspended. Both men deny the charges.
In August, a third report said Mr Sevan took nearly $150,000 in cash bribes.
The panel recommended that UN Secretary General Kofi Annan lift Mr Sevan's UN immunity for the "purposes of a criminal investigation".
Q: Was Kofi Annan involved?
It emerged last year that Mr Annan's son Kojo received payments from a company, Swiss-based Cotecna, which was contracted to monitor the OFF programme.
Mr Volcker's panel found "significant" questions over the integrity of Kojo Annan's dealings.
However, it found no evidence that the secretary general had made personal gain or influenced the Cotecna contract.
It did find that Kofi Annan failed to exert control over the programme.
"The report is critical of me personally and I accept its
criticism," he said, adding that the
findings were "deeply embarrassing to us all".
Q: Has anyone been charged over the affair?
Samir Vincent, 64, an Iraqi American, was the first to be charged in the US justice department inquiry. In January 2005, he pleaded guilty to being an illegal agent for Saddam Hussein's government and helping to skim money from the programme.
Three more people were indicted on 14 April 2005. A Texas businessman, a Bulgarian and a British citizen are accused of taking part in an alleged scheme to pay bribes to the former Iraqi regime as part of the programme.
Alexander Yakovlev, a Russian former UN procurement officer, was accused by the Volcker panel of accepting bribes of close to $1m. He has pleaded guilty and pledged to help investigators.
Q: What has the US Senate inquiry found?
A report from a Senate committee has claimed that two politicians, British MP George Galloway and former French government minister Charles Pasqua, were given allocations of oil by Iraq under the OFF programme.
They both deny the charges.
On 16 May 2005, a second set of reports accused Russian politician Vladimir Zhirinovsky, and former presidential aides Alexander Voloshin and Sergei Issakov, of receiving allocations.
But none of the committee's evidence suggests direct contacts between the three Russians and the Saddam Hussein regime over oil. Nor does the committee present evidence that any of the five men actually received any financial gain.
The US committee says that it has found documents in the Iraqi oil ministry listing contracts in the OFF programme. It has also interviewed senior Iraqi officials including former Vice-President Taha Yassin Ramadan and former Deputy Prime Minister Tariq Aziz.
Q: What outside factors are affecting the investigations?
Correspondents say the scandal must be seen within the context of the row that erupted between the US and the UN over Iraq, and the debate over the future of the UN.
"Neo-conservative" opponents of the UN in the US have held up the scandal as a symbol of the UN's incompetence and last December Republican Senator Norm Coleman - who chairs the main Senate committee investigating the scandal - became the first to call on Mr Annan to quit.
The controversial new US ambassador to the UN, John Bolton, said the Volcker report "unambiguously rejects the notion that business as usual at the United Nations is acceptable".
But UN supporters say it is inevitable that in running such a large operation mistakes occur.
Correspondents point out that billions of dollars are unaccounted for in the Iraq reconstruction fund set up to replace the OFF programme - which was under the stewardship of the US-led coalition in Iraq.
Mr Annan has also alleged that that the US and UK turned a blind eye to oil smuggling by Saddam Hussein's regime, something they reject.
The UN chief suggested London and Washington had inadequately policed UN sanctions against Iraq, enabling the regime to earn huge amounts in illegal deals.