Iranian-born Grand Ayatollah Ali Sistani is widely regarded as the spiritual leader of Iraq's Shia Muslims. He did not go into exile and spent long years under house arrest, but is now being challenged by younger, more radical leaders.
Days after his return, his home in Najaf was surrounded by armed men demanding that he leave Iraq.
During the war in Iraq, Ayatollah Sistani is believed to have issued an edict urging Iraqis not to resist coalition forces.
Sheikh Assad al-Faeli, a senior Shia cleric, has returned from Iran after two decades of exile.
He claims control of Badra, a town near the Iranian border.
Sheikh Faeli, who says he heads the Islamic Movement for Kurdish Shia, has said that he thanked the United States for getting rid of Saddam Hussein, but that US forces were not welcome to stay indefinitely.
Mohammed Bahr al-Ulloum, a Shia cleric from Najaf, is a highly respected religious scholar viewed as a liberal. He fled Iraq in 1991 after several members of his family were killed by Saddam Hussein's regime.
Mr Ulloum is a member of the Iraqi Governing Council, although he suspended his participation after a large car bombing in Najaf killed the Shia cleric Ayatollah Mohammad Baqr al-Hakim.
He said the council had failed to influence US policy on security. Mr Ulloum also threatened to set up Iraqi armed militias to secure Shia holy sites.
Ahmad el-Kebeisey, an exiled Sunni cleric, has returned to Baghdad to become a leader of Friday prayers in the Abi Hanifa mosque in al-Azamia, a Sunni district of Baghdad. He opposes the US and British presence and has instigated a protest march.
Dr Kebeisey is a former Professor of Islamic Studies at Baghdad University and was in exile in Dubai.