More than 20,000 foreign fighters are participating in the Syria and Iraq conflict.
Many of those are from western democratic countries like Australia, New Zealand, Austria and France.
This week's Dateline gives exclusive access to foreign fighters on the frontline in Syria, including Australian Abu Sulayman Muhajir.
The former preacher from Sydney is now a leading figure in al-Qaeda’s Syrian arm Jabhat al-Nusra, which has a deadly rivalry with Islamic State.
Across Western Europe, an estimated 3,500-4,100 people from 14 countries had travelled to join the conflict in Iraq and Syria, the latest figures from the International Centre for the Study of Radicalisation and Political Violence (ICSR) says.
Those estimates were made in January, and at the time ICSR said up to 150 people from land-locked Austria had joined the conflict.
The current figure is closer to 190.
While radicalisation is an issue for Austria, the ICSR has estimated even higher rates in the Balkans and in parts of Western Europe, like Belgium, Denmark and France, compared to those countries' populations.
To find these rates, SBS compared estimated fighter numbers to countries' latest population figures on the World Bank data website.
Dateline also recently reported on the radicalisation of teenagers by IS in Europe - watch The Boy Who Says No: