This report builds on ISD’s research into the Islamist foreign fighter and female migrant phenomenon, and is based on a dataset of 300 anti-ISIS foreign fighters. It sheds light on this understudied group, examining who they are, who they are fighting with, and why they are prepared to participate in a far-away conflict. It also considers the legal frameworks of the countries providing the largest numbers of anti-ISIS fighters and the possible future implications for the fighters themselves, as well as their national governments.