Iran's Islamic Revolution Guards Corps (IRGC) was set up shortly after the 1979 Iranian revolution to defend the country's Islamic system, and to provide a counterweight to the regular armed forces.
It has since become a major military, political and economic force in Iran, with close ties to the Supreme Leader, Ayatollah Ali Khamenei, and President Mahmoud Ahmadinejad, a former member.
The force is estimated to have 125,000 active troops, boasts its own ground forces, navy and air force, and oversees Iran's strategic weapons.
It also controls the paramilitary Basij Resistance Force and the powerful bonyads, or charitable foundations, which run a considerable part of the Iranian economy.
The Revolutionary Guards' power and influence are such that the US government has designated it a "proliferator of weapons of mass destruction" and its elite overseas operations arm, the Quds Force, a "supporter of terrorism".
Guardians of the Revolution
Before the 1979 revolution, Shah Muhammad Reza Pahlavi relied on military might to ensure national security and to safeguard his power.
Afterwards, the new Islamic authorities, headed by Ayatollah Ruhollah Khomeini, realised they too needed a powerful force committed to consolidating their leadership and revolutionary ideals.
The clerics therefore produced a new constitution that provided for both a regular Military (Artesh), to defend Iran's borders and maintain internal order, and a separate Revolutionary Guard (Pasdaran), to protect the country's Islamic system.
In practice, these roles have often overlapped, with the Guards also helping to keep public order and developing its own army, navy and air force.
Despite having 200,000 fewer troops than the regular military, the Guards are considered the dominant military force in Iran and are behind many of the country's key military operations.
In March 2007, it was the Guards' navy which sparked a diplomatic stand-off with the UK by detaining 15 British sailors and marines patrolling the mouth of the Shatt al-Arab waterway separating Iran and Iraq.
The US has also accused the Guards' 15,000-strong overseas operation arm, the Quds Force, of supplying explosively formed projectiles (EFPs) - powerful roadside bombs - to Shia militants in Iraq.
The force is believed to have staff in embassies around the world, from where it allegedly conducts intelligence operations and organises training camps and arms shipments for foreign militant groups which Iran supports, such as Hezbollah.
The Guards also have a powerful presence in civilian institutions, and control the Basij Resistance Force, an Islamic volunteer militia of about 90,000 men and women.
The Basij, or Mobilisation of the Oppressed, are loyalists to the revolution who are often called out onto the streets at times of crisis to use force to dispel dissent.
Such popular power, combined with the strong support of the Supreme Leader, has also made the Guards a key player in Iranian politics.
Ayatollah Ali Khamenei - also commander-in-chief of the armed forces - is believed to have used his power to expand his and the Guards' influence by appointing several former members to top political posts and using the force to suppress dissidents and reformists.
Soon after his election in 2005, President Ahmadinejad named several former veterans to key ministries in his cabinet.
After his disputed re-election in June, the Revolutionary Guards warned demonstrators against further protests.
Many people in Iran saw the subsequent crackdown on the opposition as an assertion of control by the Revolutionary Guards.
It is an impression the Guards have confirmed themselves, and members of the Basij militia, a group affiliated with the Guards, have been prominent in putting down the opposition protests.
There are also reports that the Revolutionary Guards have increased their already substantial stake in Iran's economy, with the purchase of a majority stake in the main telecommunications company.
The Guards are thought to control around a third of Iran's economy through a series of subsidiaries and trusts.
The Guards' engineering wing, Khatam-ol-Anbia (also known by an acronym, GHORB), has been awarded several multi-billion-dollar construction and engineering contracts, including the operation of Tehran's new Imam Khomeini international airport.
The Guards are also said to own or control several university laboratories, arms companies and even a car manufacturer.