Newsbeat's guide to... Iran Elections 2013
Nearly 50.5 million Iranians are heading to the polls to vote for a new president.
Iran is ruled for life by it supreme leader Ayatollah Ali Khamenei but every four years a president is chosen to run the country's day to day affairs.
For the last eight years it's been President Mahmoud Ahmadinejad in charge. But no president can stay on after eight years so it's time for someone new.
How are new candidates chosen
Theoretically anyone who's Iranian can apply but there is a vigorous vetting process carried out by one of the most powerful and influential institutions in Iran - the Guardian Council. The 12 members are unelected but they get to decide who runs for the presidency.
Eight contenders have been chosen out of hundreds. Two have dropped out. One has been disqualified. Most of them are loyal supporters of the Ayatollah.
The candidates don't represent political parties in the conventional sense but support different movements within the system.
Who's likely to win?
Saeed Jalili is Iran's top negotiator on matters to do with the country's nuclear programme and is very close to the Ayatollah.
Seen as a hero after losing a leg in the Iran-Iraq war, Jalili is a strong defender of Iran's current robust foreign policy. This includes the right to develop nuclear energy.
Hassan Rowhani is Glasgow educated and a former nuclear negotiator. He is a moderate Muslim and rejects extremism in the name of Islam.
He's being backed by Iranian reformists - those who are critical of the current Islamic system and want a change.
Rowhani has called for greater freedoms which have attracted young voters.
Last time there were accusations of vote rigging?
In 2009, thousands took to the streets after Mahmoud Ahmadinejad won a second term. Voters claimed that his challenger reformist, Mir Hossein Mousavi, who remains under house arrest, was the real winner.
Protests are banned in Iran so it was the biggest challenge to officials since the Islamic revolutions in 1979.
Many Iranians inside the country have already claimed they're struggling with slower internet speeds in the run up to this year's elections. They believe officials are trying to stop them from organising any protests but that's been denied.
Could a new leader change nuclear ambitions?
Foreign policy is ultimately determined by the Ayatollah and the Islamic Revolutionary Guards Corps (IRGC) - Iran's most powerful security and military organisation.
Iran's foreign minister has already said the candidates' views will not impact Iran's foreign policy after the election. So the current stand-off with the West over developing nuclear, is likely to continue.
What's the point of a new president then?
A new president can change the image and direction of a country because of the international profile they have.
The new president will take charge of economic policy. Iran's economy is in its worst state for decades, with high inflation, rising unemployment and negative growth. This has largely been brought about by an international ban on the purchase of Iranian oil. That was put in place to punish Iran's nuclear ambitions.
So for voters, turning the country round, will be the main issue on their mind when they decide who gets their vote.
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