One of the difficult tasks in the social sciences is integrative, interdisciplinary work. There are many commonalities across the social sciences in method, theory, and policy. The study of gangs has a tradition in the U.S. that dates back nearly 100 years, with an emerging focus in Europe and other parts of the world. This Research Note argues that there is considerable overlap between the study of gangs and that of radicalized groups. Both fields examine violence conducted largely in a group context. Group structure, demographics, marginalization, strength of membership bonds, leaving the group, and the role of prison in expanding membership are all issues the two have in common. There are lessons those who study radicalized groups can take from the long tradition of gang research. This Research Note identifies eleven lessons learned (mistakes and successes) from the study of gangs that have relevance to the study of radicalized and extremist groups.