We conceptualize political radicalization as increasing extremity of beliefs, feelings, and actions in support of intergroup conflict, and we identify mechanisms of radicalization for individuals, groups, and mass publics. Contrary to conventional wisdom, radicalization cannot be understood by focusing only on radicalized actors; most of the mechanisms identified depend on a trajectory of action and reaction that develops between those radicalized and the enemy they are radicalized against. The foundations of this trajectory are ingroup identification and perceived threat to the ingroup. We conclude that understanding radicalization requires understanding the dynamics of intergroup conflict as the conflict unfolds over time.