Radicalization and violent extremism are pressing problems across the world. After initially addressing problems in defining radicalization and extremism, this article sketches a new social-developmental model based on a systematic integration of theories and empirical findings. We propose a three-step model of radicalization starting with ontogenetic social-developmental processes during the most dynamic period for social development, from early childhood to late adolescence. These processes include the interaction of societal, social, and individual risk and protective factors. In adverse cases this interplay encourages the establishment of proximal radicalization processes between early adolescence and middle adulthood. We assume that four interrelated but distinct social-developmental processes are central conditions for radicalization and extremism: identity problems, prejudice, political or religious ideologies, and antisocial attitudes and behavior. These proximal processes are triggered by actual societal, social, or individual conflicts (such as economic crisis, victimization) and marked by continuous intergroup processes. The more intense the proximal processes, the greater the likelihood of extremist attitudes and behavior. The article closes by discussing implications for early prevention and an outlook for further research.