Ayatollah Khamenei is the supreme leader of Iran, the country's most powerful figure.
A key figure in the Islamic revolution, he was president of Iran from 1981-1989 before becoming supreme leader, a title he has for life. As supreme leader, he appoints the head of the judiciary, six of the 12 members of the Guardian Council and the commanders of the armed forces. He also confirms the president's election.
President Mahmoud Ahmadinejad
Before his surprise presidential win in 2005, Ahmadinejad, 53, was mayor of Tehran and had served in provincial political roles. He holds a doctorate in civil engineering and traffic planning.
The son of a blacksmith, Ahmadinejad's family moved from north-central Iran to Tehran when he was an infant. Some reports say Ahmadinejad was involved in the 444-day occupation of the U.S. Embassy following the 1979 Islamic Revolution, but any role remains unclear.
Ahmadinejad has become a lightning rod for international criticism for comments that include questioning the extent of the Holocaust and calling for Israel to be "wiped off the map." He has strongly defended Iran's right to uranium enrichment as part of its nuclear program, deepening the impasse with Western nations that fear Iran could be seeking nuclear weapons. Iran says it only seeks peaceful reactors for electricity.
Mir Hossein Mousavi
Mousavi served as Iran's last prime minister from 1981 to 1989, when the post was eliminated. Before entering the presidential race, he was widely identified with his leadership role during the hardships and bloodshed of the 1980-88 war with Iraq.
Some critics also question the depth of his pro-reform platform, noting that he was part of the early power structure after the Islamic Revolution that clamped down on dissent and forced ultraconservative rules such as banning music. But Mousavi, 68, later had a central role in the move toward greater political and social freedoms that began with the election of President Mohammad Khatami in 1997.
Mousavi currently serves as president of the Iranian Academy of Arts and is an amateur artist. He also is a member of the Expediency Council, which mediates disputes between parliament and the ruling clerics.
A native of northeastern Iran, Mousavi studied architecture in Tehran and was editor for the official newspaper after the 1979 Islamic Revolution. He is married to Zahra Rahnavard, a former chancellor of Al-Zahra University in Tehran until being forced out by conservatives in 2006. Both Mousavi and Rahnavard served as advisers to Khatami.
The Revolutionary Guard and the Militias
Iran's Islamic Revolution Guards Corps was set up after the revolution to defend the country's Islamic system. It is separate from the Iran's other armed forces, though they share a joint commander.
It has since become a major military, political and economic force in Iran, and the group has ties to Supreme Leader Ayatollah Ali Khamenei and President Mahmoud Ahmadinejad, who is a former member.
The Revolutionary Guard also controls the Basij Resistance Force, an Islamic volunteer militia. They are often called out onto the streets at times of crisis to use force to dispel dissent.
Clerics play a major role in Iranian society.
They are the only ones who can be elected to the Assembly of Experts, which appoints the supreme leader, monitors his performance and can theoretically remove him if he cannot fulfill his duties. They also dominate the judiciary, which is based on Islamic law.