In this article, we review the literature and present a model of radicalization and de-radicalization. In this model, we distinguish three phases in radicalization: (1) a sensitivity phase, (2) a group membership phase and (3) an action phase. We describe the micro-level, meso-level and macro-level factors that influence the radicalization process in these three phases. However, not all people become increasingly radical — they may also de-radicalize. We specify the micro-level, meso-level and macro-level factors in de-radicalization. We highlight the importance of the role of group membership and intergroup relations in the radicalization process.