Started as an al Qaeda splinter group.
Also known as Islamic State in Iraq and the Levant and Islamic State (IS).
The aim of ISIS is to create an Islamic state across Sunni areas of Iraq and in Syria.
ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts. It has taken over large swaths of northern and western Iraq.
The group currently controls hundreds of square miles. It ignores international borders and has a presence from Syria's Mediterranean coast to south of Baghdad. It rules by Sharia law.
ISIS's initial strategy for revenue was through extortion and robbery. Recently, al-Baghdadi's strategy shifted to generating resources through large-scale attacks aimed at capturing and holding territory.
Unable to serve under the new Iraq government after Saddam Hussein's military was disbanded, former Iraqi soldiers became ISIS fighters, according to Middle East expert Fawaz Gerges.
Abu Bakr al-Baghdadi: Very little is known about al-Baghdadi, but a biography posted on jihadist websites in 2013 said he earned a doctorate in Islamic studies from a university in Baghdad.
He formed the militant group in Salaheddin and Diyala provinces north of the Iraqi capital before joining al Qaeda in Iraq.
Al-Baghdadi was detained for four years in Camp Bucca, which was a U.S.-run prison in southern Iraq. He was released in 2009.
After ISIS declared the creation of the so-called "Islamic State," he began using the name Al-Khalifah Ibrahim, and now goes by that name with his followers.
2004 - Abu Musab al-Zarqawi establishes al Qaeda in Iraq (AQI).
2006 - Under al-Zarqawi, al Qaeda in Iraq tries to ignite a sectarian war against the majority Shia community.
June 7, 2006 - Al-Zarqawi is killed in a U.S. strike. Abu Ayyub al-Masri, also known as Abu Hamza al-Muhajer, takes his place as leader of AQI.
October 2006 - AQI leader Abu Ayyub al-Masri announces the creation of Islamic State in Iraq (ISI), and establishes Abu Omar al-Baghdadi as its leader.
April 2010 - Abu Bakr al-Baghdadi becomes leader of ISI after Abu Omar al-Baghdadi and Abu Ayyub al-Masri are killed in a joint U.S.-Iraqi operation.
April 8, 2013 - ISI declares its absorption of an al Qaeda-backed militant group in Syria, Jabhat al-Nusra, also known as the al-Nusra Front. Al-Baghdadi says that his group will now be known as Islamic State in Iraq and the Levant (ISIS).
April 2013 - Al-Nusra Front leader Abu Mohammed al-Jawlani rejects ISIS's attempt to merge with the group.
February 3, 2014 - Al Qaeda renounces ties to ISIS after months of infighting between al-Nusra Front and ISIS.
May 2014 - ISIS kidnaps more than 140 Kurdish schoolboys in Syria, forcing them to take lessons in radical Islamic theology.
June 9, 2014 - Monday night into Tuesday, militants seize Mosul's airport, its TV stations and the governor's office. ISIS frees up to 1,000 prisoners.
June 10, 2014 - ISIS takes control of Mosul.
June 11, 2014 - ISIS takes control of Tikrit.
June 21, 2014 - ISIS takes control of Al-Qaim, a town on the border with Syria, as well as three other Iraqi towns.
June 28, 2014 - Iraqi Kurdistan restricts border crossings into the region for refugees fleeing the fighting.
June 29, 2014 - ISIS announces the creation of a caliphate (Islamic state) that erases all state borders, making al-Baghdadi the self-declared authority over the world's estimated 1.5 billion Muslims. The group also announces a name change to the Islamic State (IS).
June 30, 2014 - The United Nations announces that an estimated 1.2 million Iraqis have been forced from their homes.
June 30, 2014 - The Pentagon announces the United States is sending an additional 300 troops to Iraq, bringing the total U.S. forces in Iraq to nearly 800. Troops and military advisers sent to Iraq are there to add security to the U.S. Embassy and the airport in Baghdad, and to provide support