Politically, religiously, ethnically, and otherwise motivated forms of extremism, radicalization, and terrorism are most important topics in current crime policy and practice. In recent times, publications, practical measures, and research on these topics exponentially increased. This article contains information about basic conceptual and methodological issues, theoretical explanations, risk factors and assessment, protective factors and resilience, and programmes of prevention and intervention. The discussion of empirical findings refers to both scientific and practical needs. Although there has been advancement in knowledge about risk assessment, protective mechanisms, and disengagement from and prevention of extremism and terrorism, there are still substantial knowledge gaps. Instead of too hectic investments into short-term oriented research and practice, more continuous and long-term approaches are necessary. Criminological and forensic psychology make important contributions in this field.