Introduction.- Foreign Fighters' in the Syria and Iraq Conflict: Statistics and Characteristics of a Rapidly Growing Phenomenon.- Foreign Fighters' Involvement in National and International Wars: A Historical Survey.- Foreign Fighters as a Challenge for International Relations Theory.- Foreign Fighters: Motivations for Travel to Foreign Conflicts.- The Emerging Role of Social Media in the Recruitment of Foreign Fighters Gabriel Weimann.- Analysing the Recruitment and Use of Foreign Men and Women in ISIL through a Gender Perspective.- The Military Impact of Foreign Fighters on the Battlefield: the Case of the ISIL.- The Status of Foreign Fighters under International Humanitarian Law Emanuele Sommario.- Foreign Fighters and International Criminal Law.- Child Soldiers: the Expanding Practice of Minors Recruited to Become Foreign Fighters.- Armed Opposition Groups' (and Foreign Fighters') Abidance by Human Rights Law: the Issue of Compliance in Syria and Iraq.