The four subject samples in this study involved a quantitative dataset of U.S.-based extremists; a quantitative dataset representative of adolescent and adult U.S. gang members; qualitative life histories of extremists based in the United States; and qualitative interview data from current and former U.S.-based gang members. The comparison of the quantitative dataset generally found clear differences between gang members and domestic extremists. One of the most striking differences between these two samples was the age of initial group involvement. Extremists with a history of gang involvement were, on average, 4 years younger at the age of group involvement compared with extremists without a gang history. Regarding basic demographics, extremists were more likely to be male than the gang sample, which was nearly one-third female. Also, domestic extremists more closely approximated the racial/ethnic composition of the United States at large; gang members, on the other hand, more closely reflected the composition of millennials, another function of the age differences between gang members and extremists. Regarding religious orientation, extremists were more likely to be Muslim or Jewish. Also, extremists had more college experience than gang members and had lower rates of poverty in both childhood and adulthood. Based on the findings of differences and similarities between gang-member and extremist characteristics. various features of interventions to prevent violent gang and terrorist behaviors are discussed. Project-related publications are listed.