The group, which U.S-led forces are bombing in Iraq and Syria, is made up of thousands of fighters from both countries as well as foreign recruits from around the world. Its leadership draws from militants with combat experience in Iraq.
Here are some of the group's main figures:
ABU BAKR AL-BAGHDADI, LEADER
Born in 1971, Baghdadi comes from an Iraqi family of preachers and Arabic teachers, according to a biography distributed on militant forums that says he studied at the Islamic University in Baghdad.
According to U.S. media reports, Baghdadi was detained for several years at Camp Bucca, a U.S-run prison in southern Iraq, before becoming head of the militant group “Islamic State of Iraq” in 2010, a predecessor to “Islamic State”, which expanded into Syria in 2013.
In June this year the growing group named Baghdadi as "caliph for the Muslims everywhere," and called on all to pledge allegiance to him. Although he is rarely pictured, a video released in July claimed to show him preaching in a mosque in Iraq's Mosul city, dressed in a black robe and turban.
He has proved ruthless in eliminating opponents and showed no hesitation in turning against former allies: He launched a war against al Qaeda's Syria wing Nusra Front, leading to a split with al Qaeda leader Ayman al-Zawahri, earlier this year.
A recent pamphlet released by ISIS claimed Baghdadi's purported lineage to the Prophet Mohammad and listed his military achievements. The United States is offering $10 million for information leading to the location, arrest, or conviction of Baghdadi, whose real name is Ibrahim Awwad Ibrahim Ali al-Badri al-Samarai.
ABU MOHAMMAD AL-ADNANI, OFFICIAL SPOKESMAN
Born in 1977 in Idlib, Syria, Adnani has delivered ISIL's main messages, including its declaration of a caliphate, which was distributed in five languages.
The United States designated him a "global terrorist" this year and says he was one of the first foreign fighters to oppose U.S-led coalition forces in Iraq since 2003 before becoming spokesman of the militant group.
A biography posted on militant websites says the man, whose real name is Taha Subhi Falaha, grew up with a "love of mosques" and was a prolific reader.
ABU MUSLIM AL-TURKMANI, SENIOR COMMANDER IN IRAQ
A former general under Saddam, former Iraqi dictator, he is believed to have taken charge of provinces captured by ISIL in Iraq. In the Iraqi military under Saddam he served in military intelligence and the republican guard.
His real name is said to be Fadel Ahmad Abdullah al-Hiyali and he was also imprisoned at Camp Bucca in Iraq. Former Saddam Baathists in Iraq have joined forces with ISIL to fight the Baghdad government.
ABU OMAR AL-SHISHANI, SENIOR COMMANDER IN SYRIA
Born in Georgia in 1986, Shishani "the Chechen" has been described as ISIS's military commander in Syria and led an offensive to capture a large area of land up to the Iraqi border.
His real name is Tarkhan Batirhvili, and he joined fighters battling the Syrian army in 2012 and swore allegiance to Baghdadi.
Syrian Rebels and Kurdish forces say he is leading ISIS's main strike force and has headed a recent offensive to capture a Kurdish town close to the Turkish border.
Shishani, who sports a striking ginger beard, is frequently shown on the battlefield, and has been pictured receiving military vehicles seized in Iraq and brought into Syria.