While women have been active members of terrorist organizations for years, the prominence that the question of women in terrorism has achieved as a result of women’s involvement in al-Qaeda is unprecedented. There has since been a much-welcomed increase in focus on women’s roles in al-Qaeda and more recently the so-called Islamic State. Yet, even with all the recent attention on Islamic State, up to 2018 there remained a lack of evidence base for determining just what proportion of foreign affiliates women constituted – a full five years after foreigners started travelling to Iraq and Syria. This chapter seeks to rectify that. Drawing upon recent research, this chapter shows that female recruits to Islamic State constituted the largest recorded number of women who have travelled or ‘migrated’ to join any terrorist organization in history, and demands attention today in terms of policy and practice-oriented responses to managing these women, including the prevention of future support for the ideology. Drawing on the case study of Islamic State, this chapter will consider the motivations of women that drove them to join the organization, and the roles they held in Iraq and Syria. It will next consider potential security implications of these women today, and finally discuss the policy and practice-oriented implications this population presents. While women in IS cannot be said to be fully representative of women in terrorism today, they demonstrate an important population of interest, and a new trend in how women are engaging in international jihadist militancy.