The concept of 'radicalization' is now used to account for all forms of violent and non-violent political Islam. Used widely within the security services and picked up by academia, the term was initially coined by the General Intelligence and Security Service of the Netherlands (AIVD) after the 9/11 and Pentagon attacks, an origin that is rarely recognised. This book comprises contributions from leading scholars in the field of critical security studies to trace the introduction, adoption and dissemination of 'radicalization' as a concept. It is the first book to offer a critical analysis and history of the term as an 'empty signifier', that is, a word that might not necessarily refer to something existing in the real world. The diverse contributions consider how the term has circulated since its emergence in the Netherlands and Belgium, its appearance in academia, its existence among the people categorized as 'radicals' and its impact on relationships of trust between public officials and their clients. Building on the traditions of critical security studies and critical studies on terrorism, the book reaffirms the importance of a reflective approach to counter-radicalization discourse and policies. It will be essential reading for scholars of security studies, political anthropology, the study of Islam in the west and European studies.