Grand Ayatollah Ali Sistani is the prime marja, or spiritual reference, for Shia Muslims everywhere.
He is one of only five living grand ayatollahs and the most senior Shia cleric in Iraq.
The reclusive cleric left Najaf for the first time in years for heart surgery
During the regime of Saddam Hussein, the elderly cleric lived in uneasy stalemate with the status quo. He spent long periods under house arrest but avoided overt political activity.
The low-profile approach he had to adopt to survive in Iraq has been criticised by younger, more radical Shia leaders such as Moqtada Sadr.
In April 2003, just after the fall of the regime, club-wielding members of the Sadr Group besieged Ayatollah Sistani's house, demanding that he leave the country and that he recognise Moqtada Sadr as a marja.
The ayatollah went into hiding - but he since re-emerged to wield enormous power over Iraq's Shia majority.
The Persian-born ayatollah represents the conservative and mainstream of Iraqi Shias - rejecting the model of Iranian-style theocracy in favour of a separation between religion and politics.
In the first few months after the war, Ayatollah Sistani adopted a quiet approach.
Coalition officials, aware of his influence, repeatedly praised him for his moderate views.
Ayatollah Sistani rarely made political statements and has repeatedly urged his Shia followers not to take up arms against the occupation forces.
But in October and November 2003 the ayatollah became increasingly critical of the US political plans for Iraq.
He rejected a US plan allowing for the transfer of sovereignty to an unelected provisional government in June 2004.
"We want free elections and not appointments," he said.
He is thought to be particularly concerned that Iraq's majority Shia might be disenfranchised again, just as they were after the uprising against British occupation in the 1920s.
In January, thousands of his supporters took to the streets backing his call for direct elections.
Ayatollah Sistani did finally accept the recommendations of a UN mission which concluded that elections could not be held before the June 2004 power handover deadline.
But he embarrassed the US by instigating the walkout of five Shia members of the governing council at the elaborate signing ceremony for the country's interim constitution.
Clashes across southern Iraq between Moqtada Sadr's Mehdi Army militia and US forces in April took the spotlight away from Ayatollah Sistani's criticisms of the handover process.
However, he played a pivotal role in reaching a ceasefire that halted the fighting in May.
And when it did materialise, Ayatollah Sistani gave cautious approval to the new caretaker government, though he called on it to prove its efficiency.
In August, the famously reclusive cleric, who has not left his sparsely-furnished base in Najaf for years, suddenly flew to London to receive treatment for a heart condition.
Fighting broke out again in the holy city almost simultaneously, between Sadr's militia and multi-national troops, and when Ayatollah Sistani returned three weeks later a huge responsibility was placed on his shoulders to bring an end to the bloodshed.