Scholars of political terrorism generally agree that the radical group is usually a splinter faction of an opposition movement. Seldom, however, is an attempt made to incorporate insights and tools from the literature on social movements and contentious politics into the study of the process by which a faction splinters from the larger opposition movement and adopts terrorist tactics-a process commonly known as radicalization. Drawing upon the relational approach from the literature on contentious politics, this article seeks to further understanding of radicalization by examining how and when relational mechanisms, operating in their respective relational arenas, interact and combine to drive it. Proposed is a relational framework for a comparative analysis of radicalization at three levels-domestic, ethno-national, and international-employing the case of the Weather Underground, Fatah-Tanzim, and al-Qaeda respectively.