Islamic State stands with al-Qaeda as one of the most dangerous jihadist groups, after its gains in Syria and Iraq.
Under its former name Islamic State in Iraq and the Levant (Isis), it was formed in April 2013, growing out of al-Qaeda in Iraq (AQI).
It has since been disavowed by al-Qaeda, but has become one of the main jihadist groups fighting government forces in Syria and Iraq.
Its precise size is unclear but it is thought to include thousands of fighters, including many foreign jihadists.
The organisation is led by Abu Bakr al-Baghdadi. Little is known about him, but it is believed he was born in Samarra, north of Baghdad, in 1971 and joined the insurgency that erupted in Iraq soon after the 2003 US-led invasion.
In 2010 he emerged as the leader of al-Qaeda in Iraq, one of the groups that later became Isis.
Baghdadi is regarded as a battlefield commander and tactician, which analysts say makes his group more attractive to young jihadists than al-Qaeda, which is led by Ayman al-Zawahiri, an Islamic theologian.
Prof Peter Neumann of King's College London estimates that about 80% of Western fighters in Syria have joined the group.
IS claims to have fighters from the UK, France, Germany and other European countries, as well as the US, the Arab world and the Caucasus.
Unlike other rebel groups in Syria, IS is seen to be working towards an Islamic emirate that straddles Syria and Iraq.
The group has seen considerable military success. In March 2013, it took over the Syrian city of Raqqa - the first provincial capital to fall under rebel control.
In January 2014, it capitalised on growing tension between Iraq's Sunni minority and Shia-led government by taking control of the predominantly Sunni city of Fallujah, in the western province of Anbar.
It also seized large sections of the provincial capital, Ramadi, and has a presence in a number of towns near the Turkish and Syrian borders.
The group has gained a reputation for brutal rule in the areas that it controls.
However, it was its conquest of Mosul in June that sent shockwaves around the world.
The US said the fall of Iraq's second city posed a threat to the entire region. It may also have made ISIS the most cash-rich militant group in the world.
Initially, the group relied on donations from wealthy individuals in Gulf Arab states, particularly Kuwait and Saudi Arabia, who supported its fight against President Bashar al-Assad.
Today, IS is said to earn significant amounts from the oil fields it controls in eastern Syria, reportedly selling some of the supply back to the Syrian government. It is also believed to have been selling looted antiquities from historical sites.
Prof Neumann believes that before the capture of Mosul in June 2014, IS had cash and assets worth about $900m (£500m). Afterwards, this rose to around $2bn (£1.18bn).
The group reportedly took hundreds of millions of dollars from Mosul's branch of Iraq's central bank. And its financial windfall looked set to continue if it maintains control of oil fields in northern Iraq.
The group has been operating independently of other jihadist groups in Syria such as the al-Nusra Front, the official al-Qaeda affiliate in the country, and has had a tense relationship with other rebels.
Baghdadi sought to merge with al-Nusra, which rejected the deal, and the two groups have operated separately since.
Zawahiri has urged IS to focus on Iraq and leave Syria to al-Nusra, but Baghdadi and his fighters openly defied the al-Qaeda chief.
Hostility to IS grew steadily in Syria as regularly attacked fellow rebels and abused civilian supporters of the Syrian opposition.
In January 2014, rebels from both Western-backed and Islamist groups launched an offensive against IS, seeking to drive its predominantly foreign fighters out of Syria.
Thousands of people are reported to have been killed in the infighting.