Ayatollah Ali Khamenei is Iran's spiritual leader and highest authority. His veto is final in Iranian political affairs.
He is widely regarded as the figurehead of the country's conservative establishment and has been described as one of the three defining influences of the revolution.
Despite Western focus on President Ahmadinejad since his taking office in 2005, the most important figure in Iran is Ayatollah Khamenei in his role as Supreme Leader.
Ayatollah Khamenei has repeatedly denounced the West, and in particular the United States.
However, in a speech in 2008 he said that he had "never said that the relations will remain severed forever". That no doubt gave some comfort to US President Barack Obama, who has taken the unprecedented step of making overtures to the Iranian leader.
In 1989, Ayatollah Khamenei succeeded the original Supreme Leader and founder of the Islamic Republic of Iran, Ayatollah Khomeini. Before that he was president for two successive terms from 1981-1989.
Ayatollah Khomeini was a cleric of the highest rank, a Source of Emulation. When Ayatollah Khamenei took over, the constitution had to be amended to allow the post to be held by a lower-ranking theologian.
When he was president he was often at odds with the then Prime Minister, Ali Hossein Mousavi, whom he perceived as being left-leaning.
However, as Mr Mousavi had the backing of the supreme leader, Ayatollah Khomeini, their conflicting views on economic, social and religious policies were left to fester.
One of Ayatollah Khamenei's first decisions, when he became Supreme Leader on the death of Ayatollah Khomeini, was to revise the constitution to abolish the post of prime minister.
Ayatollah Khamenei is often described as lacking the charm and popular support of his predecessor.
He brought to the position of Supreme Leader the powers and contacts he had made as president and has cemented his position by developing networks in the various institutions and security forces in Iran.
In 1997 he famously clashed with Ayatollah Hossein Ali Montazeri, a respected scholar who ranks higher in the hierarchy.
Ayatollah Montazeri, who is also one of Iran's leading dissidents, questioned the powers of the Supreme Leader. This led to the closure of his religious school, an attack on his office in Qom and to a period of house arrest.
In November 1999, Ayatollah Khamenei went on TV to defend a controversial Special Clerical Court, which had just found the editor of a leading reformist daily guilty of publishing anti-Islamic articles.
Khordad editor and former interior minister Abdullah Nuri had described the court as "illegal". Khamenei hit back, saying there was a need for a court that "had the courage to put a cleric on trial and demand answers".
As Supreme Leader, Ayatollah Khamenei has the power to select directly and indirectly members of the Guardian Council. The council is in charge of elections, both the supervision of the polls and the confirming of candidates.
In the 2004 elections it disqualified thousands of parliamentary candidates including many moderates, reformists and members of the previous government. Conservatives won 70% of the vote.
Hadi Khamenei, the Supreme Leader's brother, criticised the disqualifications, saying they distorted Iranian democracy.
But Ayatollah Khamenei has consistently backed the supervisory role of the conservative Guardian Council.
In August 2000, he sided with the Guardian Council in rejecting a Majlis (parliament) bill reforming the country's press law.
A letter he wrote to parliament, quoted by the state news agency, said the current law had prevented the "enemies of Islam" from taking over the press.
"Thus any re-interpretation of the law is not in the interests of the country," the letter argued.
The letter led to scuffles in the Majlis and to a debate on the powers of the Majlis and the Guardian Council. The press bill was withdrawn.
Ayatollah Khamenei did however intervene in the case of pro-reform academic Hashem Aghajari. In November 2002, Mr Aghajari said Muslims should re-interpret Islam rather than blindly follow leaders.
The judiciary sentenced him to death. When protests erupted in the capital, Khamenei ordered a review of the sentence which was later commuted to a prison sentence.
In May 2003, over 100 members of parliament wrote an open letter to the ayatollah, warning that unless he removed obstacles to reforms the survival of the Islamic system would be at risk.
The MPs said Iran was facing a stark choice between democracy and dictatorship. The letter was posted on two Iranian websites, but was removed by the authorities after 24 hours.
In his inaugural address as president in 1981, Ayatollah Khamenei vowed to stamp out "deviation, liberalism, and American-influenced leftists", in a statement that set the tone for his leadership.
When pro-reform students rioted in June 2003, Ayatollah Khamenei was quick to warn that such actions would not be tolerated. And he blamed the US for stirring up the trouble.
"Leaders do not have the right to have any pity whatsoever for the mercenaries of the enemy," he said in a broadcast speech.
During and after the US-led war on Iraq, he was sharply critical of Washington's policies. "The occupation of Iraq is not a morsel that the US can swallow," he said.
In 2009, when the US president offered Iran a "new beginning" of diplomatic engagement, Khamenei's response was muted.
Addressing students a few days after the Iranian New Year message, he said he had seen no change in America's attitude or policy, singling out US support for Israel and sanctions against Iran.
But he said that if President Obama altered the US position, Iran was prepared to follow suit.