The process by which extremist ideas and behaviors develops is commonly referred to as “radicalization.” This chapter reviews some of the vexing issues in defining radicalization, particularly in the absence of rigorous research, and introduces the potential for social science—at the mass-, group- and individual-level--to illuminate the field's understanding of the process. It is clear, however, that the process does not unfold in the same way for all people. Different pathways can lead to radicalization, and conversely, different persons on a shared pathway or trajectory may have different outcomes. The chapter concludes by outlining the nature of the radicalization process and reviewing some of the leading conceptual models that describe the progression.