This article addresses sex trafficking committed by the terrorist group Da’esh in the Syrian Arab Republic and its neighboring Republic of Iraq. It proposes primarily that sex trafficking perpetrated by Da’esh falls into at least two unique categories of trafficking — “combat trafficking” and “institutional trafficking” — that require markedly different legal responses in order to ensure accountability and justice. The article outlines the divergent factual patterns and characteristics of each form of trafficking, exploring how “combat trafficking” occurs through the active invasion of territory, whereby Da’esh forcibly captures women and girls and subjects them to sexual slavery, and how “institutional trafficking” occurs outside the active conflict, whereby Da’esh recruits women and girls to join the group and subjects them to forced marriage and domestic servitude. It discusses how institutional trafficking has been largely overlooked in discussions of accountability for Da'esh's trafficking crimes. This article then argues for accountability for institutional trafficking under the international system of Transnational Criminal Law (“TCL”). It delineates how the crime of trafficking has been interpreted and defined to date within this system and how that definition applies to Da’esh’s acts of institutional trafficking. It explains how TCL is best suited to addressing this form of trafficking. The article then argues that full accountability for perpetrators of this form of trafficking is two-fold, requiring recognition of the crime and enforcement of the criminalization of it.