The brutal footage of what appears to be the beheading of US journalist James Foley is not the first of its kind to appear online.
Beheadings are a long-known feature of extremism in the Middle East, and there have been reports of decapitations of Christians in Iraq recently as Islamic State fighters continue their attacks there.
While some claim passages in the Koran can be seen to direct orthodox followers to behead, others have said these need to be read in the context of historic battles and are not supposed to be taken literally.
Story continues after slideshow...
Gareth Stansfield, a professor of Middle East Politics at the University of Exeter, said the term "beheading" does not do justice to the gruesome nature of what he said now seems to be "the ritualistic hacking off of heads".
Writing on the Royal United Services Institute website recently he said the continued ruthless killings serve a purpose.
"This purpose is to bring ISIS (another name for Islamic State) not only to the attention of those people who they are seeking to subdue now, in the Middle East, but to the attention of those peoples who they may challenge in the future, in the West."
Writing in 2005 Timothy Furnish, then assistant professor of history at Georgia Perimeter College in Atlanta, said the practice of decapitation had "both Qur'anic and historical sanction" and is believed by those who use it to have been allowed by God.
In the Middle East Quarterly he said: "Islam is, for this determined minority of Muslims, anything but a 'religion of peace'. It is, rather, a religion of the sword with the blade forever at the throat of the unbeliever."Suggest a correction