This chapter characterizes violent extremism as an ideology, and associated communication-based or overt behavior, that protects, promotes, advances, and defines a group’s social identity, and is implicitly or actually violent. It presents a social identity theory and, primarily, an uncertainty-identity theory account of how normal social identity-based group and intergroup behaviors can become violently extreme. Social identity processes are driven by people’s motivation to (a) secure a favorable sense of self though belonging to high status groups, and (b) reduce uncertainty about themselves and who they are through identification with distinctive groups with unambiguously defined identities. In the former case, people strive to protect or improve their group’s status relative to other groups, and when moderate nonviolent strategies are continuously thwarted, they can reconfigure their group’s identity to incorporate and promote violent extremism. In the latter case, people strive to resolve feelings of self-uncertainty by identifying with distinctive groups, and when intergroup distinctiveness is blurred and their group’s social identity becomes fuzzy they are attracted to ethnocentrism, populist ideology, autocratic leaders, and ultimately violent extremism. The chapter ends by identifying warning signs of radicalization and intervention principles.