Islamic State (also known as Islamic State in Iraq and Syria (ISIS)) is one of the most dangerous jihadist groups. It was formed in April 2013, growing out of al-Qaeda in Iraq (AQI).
The aim of ISIS is to create an Islamic state across Sunni areas of Iraq and in Syria. It is led by Abu Bakr al-Baghdadi who is regarded as a battlefield commander and tactician.
ISIS is known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts -- has taken over large swaths of northern and western Iraq.
Its precise size is unclear but it is thought to include thousands of fighters, including many foreign jihadists.
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.
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.
On August 20, 2014, Islamic State (IS) released a video that shows the beheading of U.S. journalist James Foley, threatening the life of another American if President Barack Obama doesn't end military operations in Iraq.
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.