This article reviews some of the milestones of thinking about political radicalization, as scholars and security officials struggled after 9/11 to discern the precursors of terrorist violence. Recent criticism of the concept of radicalization has been recognized, leading to a 2-pyramids model that responds to the criticism by separating radicalization of opinion from radicalization of action. Security and research implications of the 2-pyramids model are briefly described, ending with a call for more attention to emotional experience in understanding both radicalization of opinion and radicalization of action.

After the shock of the 9/11 attacks, security officials in the United States struggled to understand the process by which individuals and groups move to terrorism. The hope was to “get to the left of the boom”—to predict and, ideally, to prevent future attacks. Radicalization came to be the word used to refer to the human developments that precede terrorist attack. This article aims to provide an overview of thinking about radicalization.

The enormous and still expanding literature on radicalization cannot be fully represented here. The first section of the article reviews milestones in terrorism research since the 9/11 attacks. Milestones were determined on the basis of contribution to psychological theorizing of radicalization...