Following the 1979 revolution in Iran led by Ayatollah Ruhollah Mousavi Khomeini, Iran was transformed into a republic system ruled by the Islamic ideology.
Established as the Islamic republic, the new ruling system held first presidential election on Jan. 25, 1980, in which Abolhassan Banisadr, a politician and economist, was elected as the first president in Iran's history.
As the president reaping 75.6 percent of the votes, he struggled against conservative rivals who sought to influence his administrative appointments.
The conflicts put Banisadr, who was the advocate of market economy, and his prime minister, Mohammad-Ali Rajaei, soon at odds as he refused to accept many of the prime minister's cabinet nominations.
Banisadr wrote a letter to Ayatollah Khomeini, and blamed the cabinet ministers for a worsening economy. The letter, as well as his opposition to Iran's holding American hostages taken from the U.S. embassy in Tehran on Nov. 4, 1979, angered the conservative-dominated parliament.
He was dismissed from office after being impeached by the parliament for incompetence on June 21, 1981, and later he fled to France.
Following this disturbing political experience in the early life of the Islamic republic, Rajaei was elected the president of Iran on Aug. 2, 1981, after serving as prime minister under Banisadr. He was also minister of foreign affairs while he was prime minister.
Rajaei was assassinated in a bombing on Aug. 30, 1981, only 28 days after assuming his presidency, along with prime minister Mohammad-Javad Bahonar.
For the second time in 1981, the Iranians went to the ballot boxes, and Ayatollah Ali Khamenei, the incumbent Supreme Leader, was picked as the third president winning over 95 percent of the votes.
Khamenei became the first cleric to serve in the office from 1981 to 1989. His term in the office fell in during the Iran-Iraq War that left millions killed and injured.
Khamenei helped guide the country during the war in the 1980s, and developed close ties with the young Islamic Revolution Guards Corps.
As the president, he had a reputation of being deeply interested in the revolutionary ideas, anti-U.S. sentiments, military development, resilient economy, and administrative details.
Akbar Hashemi Rafsanjani, another Iranian cleric and moderate politician, was elected president of Iran by an overwhelming margin shortly thereafter and was in the office from 1989 to 1997.
Rafsanjani showed political skills in promoting his pragmatic policies and favoured Iran's international engagement and renewing its ties with Europe as part of a strategy to use foreign investment and free enterprise to revive the country's war-torn economy.
Domestically, he implemented family-planning practices, in effect reversing previous policies which encouraged population growth.
In the 1997 elections, the reformist Mohammad Khatami was one of the four candidates to run for presidency and was open on social issues. With strong support from the country's youth, women, and intellectuals, he was elected the fifth president by almost 70 percent of the vote.
Some of the reformists he appointed to the cabinet were controversial, but nonetheless were approved by Iran's conservative parliament.
Gradually, however, tension between the president and conservatives grew, and, beginning in 1998, a number of key Khatami supporters were prosecuted and harassed as a result.
Khatami advocated increased contact with the United States, but his domestic opponents hindered rapprochement between the two countries. Khatami was reelected in 2001 by an overwhelming majority of the vote for another four years until 2005.
In 2005, Mahmoud Ahmadinejad announced his candidacy for presidency of Iran.
Despite his service as mayor of the capital Tehran, he was largely considered a minor political figure.
Through the supports of the conservatives and economically-deprived strata of society, however, Ahmadinejad managed to secure one-fifth of the vote, which propelled him into the second round of balloting, in which he easily defeated his more moderate rival, Hashemi Rafsanjani.
He was confirmed president on Aug. 3, 2005 by the country's Supreme Leader, Ayatollah Ali Khamenei.
Ahmadinejad presented himself as a populist, focusing on issues such as poverty and social justice. He was very active in foreign affairs, vigorously defending Iran's nuclear program against the West criticism.
He also prompted West's condemnation with comments calling for Israel to be "wiped off the map" and for labelling the Holocaust a myth.
On June 12, 2009, the presidential results indicated that Ahmadinejad had secured an outright victory, achieving more than 60 percent of the vote.
His second term, which ended in August 2013, was featured with a confrontation with the Khamenei and the parliament.
In March 2013, the moderate Hassan Rouhani entered the race for president, positioning himself as a critic of the outgoing Ahmadinejad's economic policies and confrontational approach to foreign affairs.
He also stated that he would support restarting international negotiations regarding Iran's nuclear program in hopes of removal of sanctions against Iran.
Rouhani's candidacy appeared to be a long shot until several better-known reformist and moderate candidates were disqualified or dropped out, leaving him as the only moderate left among a number of conservatives.
After collecting endorsements from high-profile figures including Rafsanjani and Khatami in the final weeks of the race, Rouhani won a commanding victory over the fragmented conservative field, taking more than 50 percent of the vote in the June 14 election and avoiding a runoff.
Following Rouhani's inauguration in August 2013, he launched a diplomatic campaign to improve Iranian engagement with the West.
Rouhani's pledge to restart international dialogue regarding Iran's nuclear activities was fulfilled in early November 2013 with the opening of talks between Iran and a group comprising the United States, China, Russia, France, Germany, and the United Kingdom.
By the end of the month, an interim agreement was reached that eased sanctions on Iran and placed a variety of restrictions on the nuclear program.
A final agreement was reached in July 2015 that required Iran to reduce its nuclear stockpile and allowed inspections of its nuclear facilities in exchange for gradual reduction of sanctions.
On April 21, 2017, the one-month race of Iranian candidates for the upcoming presidential position started to be followed by a waiting period until May 18.
Out of 1636 registered candidates, six qualified candidates, including three principalists, or conservatives as they are commonly referred to, plus three centrists and reformist, are the hopefuls for the next four-year term.
For May 19 vote, the centrists, led by the President Hassan Rouhani, are hoping not to lose their current sovereignty.

Source: Xinhua