Extremism and radicalization towards violence are urgent topics in many countries. Numerous research projects are carried out, of which many focus on risk factors only. In contrast, this article contains a systematic review of the rare international research on protective factors. After screening more than 2,000 documents, we found 17 reports containing 21 analyses that specifically addressed potential protective effects and provided quantitative data. Most studies addressed religious/ethnic extremism; far-right, far-left, and mixed forms were less frequent. Thirty different protective factors showed significant effects. Many were found in single analyses, but there were various replicated factors such as self-control, adherence to law, acceptance of police legitimacy, illness, positive parenting behavior, non-violent significant others, good school achievement, non-violent peers, contact to foreigners, and a basic attachment to society. Most findings are similar to what we know from more general research on youth violence, but there are also some protective factors that seem to be more specific, particularly with regard to religious/ethnic extremism. In conclusion, it is suggested to relate the topic of extremism and violent radicalization more strongly with other fields of developmental and life course criminology. For further progress on this path, more research on protective factors and integrated theoretical concepts are needed. This will also contribute to effective prevention.