The last two decades have seen the term ‘foreign fighter’ enter our everyday vocabulary. The insurgencies in Afghanistan and Iraq, the Syrian Civil War and the rise and fall of the Islamic State group have sparked public interest in the phenomenon of people choosing to leave their own country and fight in a foreign conflict. Foreign fighters, their origins, motives, activities and potential danger to their home countries have become subjects of debate, attracting contributions from politicians, military personnel, the media, security analysts, political scientists, legal scholars but to a much lesser extent from historians. The ten articles of this special issue showcase new historical research on foreign military labour, including an overview of the early modern period and numerous case studies which cover the last 165 years and stretch over five continents. The aim is to better understand the experiences and challenges faced by both the foreigners and the host country, particularly its armed forces, and to highlight the significance of these trends to the contemporary debate on foreign fighters. Designed after a conference held in June 2018 at the Centre for History at Sciences Po, Paris the issue is inspired by a number of key questions. What motivated individuals to join a foreign conflict or army? How were they treated and perceived by their host, and, when taken prisoners, by the enemy? How complete was their integration in the host’s combat formations? How did foreigners perform in battle? What happened to them after the war ended?