The emergence in the twenty-first century of a growing body of research into the recruitment and behaviour of foreign fighters is due to the contemporary prominence of the transnational jihad movement. This chapter assesses the relationship between foreign fighters and terrorism, and supports the existing claims that foreign fighters are a type of insurgent, and therefore distinct from domestic or international terrorists. As with locally organized insurgents or military forces, some foreign fighters engage in terrorist attacks against non-combatants. Furthermore, because they conceive of their political grievances in transnational terms, after leaving their war zones they pose a distinct – although historically small – threat of perpetrating terror attacks in their home countries or other states to which they travel. As governments and international organizations develop new anti-terrorism provisions designed to curtail foreign fighter mobilization and to prevent their return, it is important to preserve analytic frameworks for understanding the relationship between foreign fighters and terrorism.