This book offers various perspectives, with an international legal focus, on an important and underexplored topic, which has recently gained momentum: the issue of foreign fighters. It provides an overview of challenges, pays considerable attention to the status of foreign fighters, and addresses numerous approaches, both at the supranational and national level, on how to tackle this problem. Outstanding experts in the field - lawyers, historians and political scientists - contributed to the present volume, providing the reader with a multitude of views concerning this multifaceted phenomenon. Particular attention is paid to its implications in light of the armed conflicts currently taking place in Syria and Iraq. Andrea de Guttry is a Full Professor of International Law at the Scuola Superiore Sant'Anna, Pisa, Italy. Francesca Capone is a Research Fellow in Public International Law at the Scuola Superiore Sant'Anna. Christophe Paulussen is a Senior Researcher at the T.M.C. Asser Instituut in The Hague, the Netherlands, and a Research Fellow at the International Centre for Counter-Terrorism - The Hague.