Though widely used by academics and policy-makers in the context of the 'war on terror', the concept of radicalization lacks clarity. This article shows that while radicalization is not a myth, its meaning is ambiguous and the major controversies and debates that have sprung from it are linked to the same inherent ambiguity. The principal conceptual fault-line is between notions of radicalization that emphasize extremist beliefs ('cognitive radicalization') and those that focus on extremist behavior ('behavioural radicalization'). This ambiguity explains the differences between definitions of radicalization; it has driven the scholarly debate, which has revolved around the relationship between cognition and behavior; and it provides the backdrop for strikingly different policy approaches—loosely labeled 'European' and 'Anglo-Saxon'—which the article delineates and discusses in depth. Rather than denying its validity, the article calls on scholars and policy-makers to work harder to understand and embrace a concept which, though ambiguous, is likely to dominate research and policy agendas for years to come.