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.