A look at Iran's voters, presidents and election procedures:
SIZE: There are more than 50 million eligible voters in a population of about 76 million. About a third of the voters are under 30 – born after the 1979 Islamic Revolution. About 60 percent of the voters are in cities.
VOTERS: Voting age is 18, raised from 16 in 2007. Iranians abroad can vote in diplomatic compounds and other polling sites.
PRESIDENTIAL PROCESS: The Guardian Council, controlled by the ruling clerics, vets all candidates for high office. In this election, 686 names were submitted and eight were allowed to run. A simple majority – 50 percent plus 1 of votes cast – is needed for victory. Otherwise, a two-candidate runoff will be held June 21. The winner will take office in late August.
OTHER ELECTIONS: Municipal elections will also be taking place alongside the presidential vote for the first time. Nearly 350,000 candidates are registered for some 170 seats on municipal councils.
TURNOUT: Voter turnout was reported at nearly 85 percent in the disputed re-election of President Mahmoud Ahmadinejad in 2009. In 2005, turnout was reported as 63 percent in the first round and 60 percent in the second. In 2001, it was reported as 68 percent.
WHO RUNS THE ELECTION: The Interior Ministry oversees the election. No outside election observers are permitted. Protesters in 2009 claimed vote rigging denied reformist candidate Mir Hossein Mousavi victory and handed victory to Ahmadinejad. The ministry said its data indicated Ahmadinejad had strong backing in rural areas that overcame Mousavi's core support in Tehran and other cities. Mousavi and another 2009 candidate, Mahdi Karroubi, have been under house arrest since early 2011.
PAST PRESIDENTS: Abulhassan Banisadr (1980-81, dismissed by parliament); Mohammad Ali Rajai (August 1981, assassinated); Ayatollah Ali Khamenei, now Supreme Leader (1981-89); Akbar Hashemi Rafsanjani (1989-97); Mohammad Khatami (1997-2005); Mahmoud Ahmadinejad (2005-13).