Iranians will be deciding on who their president will be in May, in which incumbent pragmatist President Hassan Rouhani is seeking re-election against conservative rivals who have been gearing up for a political showdown.
Within Iran's complex mix of clerical rulers and elected officials, Supreme Leader Ayatollah Ali Khamenei has the final say on all state matters, including Tehran's foreign policy. This means that, although the president has some say, Khamenei can overrule any decisions he makes.
The Guardian Council, charged with vetting all candidates, disqualified hardline former president Mahmoud Ahmadinejad from running for the 19 May election, though observers believe that the former president nominated himself as a publicity stunt to boost the profile of his ally, Hamid Baghaei who has since been disqualified.
The following are the three main contenders out of six qualified candidates to run for the upcoming election:
PRESIDENT HASSAN ROUHANI
A mid-ranking Shia Muslim cleric, Rouhani won a landslide victory in 2013 by promising to end Iran's international isolation and create a freer society for Iranians.
Rouhani, 68, championed a nuclear deal with six major powers in 2015 that ended a more than decade-old standoff with the West. Under the deal, most sanctions imposed on Iran were lifted in 2016 in return for Tehran curbing its nuclear programme, seen by many to be an attempt to gain atomic weapons and threaten Middle Eastern security and stability.
Iranian reformists, led by former President Mohammad Khatami, have endorsed Rouhani, whose hardline rivals say he has failed to revive the economy despite sanctions being lifted.
Rouhani, also a lawyer, is the seventh president of Iran since the 1979 so-called Islamic Revolution that brought Ayatollah Ruhollah Khomeini to power.
Another of Iran's presidential candidates, Eshaq Jahangiri, Iran's first vice president, is a Rouhani ally who served as the minister of mines and industries from 1997 to 2005. Jahangiri, 60, said he threw his hat into the ring to "stand by Rouhani and complement him."
Jahangiri registered to run for the election amid concern among Rouhani's allies that the president might be disqualified by the Guardian Council. He is widely expected to drop out of the election to avoid a split of the moderate camp's votes.
A close ally of Khamenei, Raisi is seen as the main hardline candidate challenging Rouhani in the election. The influential but mid-ranking cleric in the Shia religious and political hierarchy, has been a senior official for decades in the judiciary, which enforces radical clerical control of the country.
Raisi, a law professor, has criticised Rouhani's economic record, portraying himself as a champion of the poor. He has promised a revival of the "values" of the Islamic Revolution, attempting to link his image to not only Khamenei, but Khomeini himself.
Conservatives have largely supported Raisi, appointed by Khamenei in 2016 as the custodian of Astan Qods Razavi, an organisation in charge of a multi-billion-dollar religious foundation that manages donations to Iran's holiest shrine in the northern city of Mashhad.
Critics of the hardliner point to the fact that Raisi was one of four Sharia, or Islamic law, judges who oversaw the execution of tens of thousands of political prisoners in 1988.
Some Iranian politicians believe Raisi is being groomed to succeed Khamenei and the presidency is just a first step. Khamenei served as a president for two terms before taking power in 1989 following Khomeini's death.
MOHAMMAD BAQER QALIBAF
A former police chief, Qalibaf has been the mayor of Tehran since 2005 and is viewed as a pragmatic conservative. A former member of Iran's elite Islamic Revolutionary Guard Corps (IRGC), Qalibaf has been a staunch critic of Rouhani's economic policy. He has promised to more than double Iran's income and tackle unemployment by creating five million jobs.
Qalibaf, born in 1961, lost to Ahmadinejad in 2005 and Rouhani in 2013 presidential elections.
His popularity was damaged by a corruption investigation in 2016 and the collapse of a tower in Tehran, which killed 20 firefighters.