This book showcases new historical research on foreign soldiers, including an overview of the early modern period and numerous case studies which cover the last 175 years and stretch over 5 continents. 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, political scientists, legal scholars but to a much lesser extent from historians. The ten essayss in this volume showcase new historical research on foreign military labour. The aim of the volume 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. The chapters in this book were originally published as a special issue of the journal European Review of History.