A widely accepted viewpoint among terrorism experts and counter-terrorism practitioners is that terrorism depends on the radicalization of its instigators and perpetrators. This chapter examines various definitions of radicalization with a view toward formulating a working definition that may be of use to terrorism scholars and counter-terrorism practitioners. It is concluded that the current usage of the term radicalization is problematic for the productive social scientific analysis of the motivational bases of socio-political violence since the term is relative, subjective, and value-laden. A new working definition of the term is provided.