A striking number of jihadi terrorists grew up in the United States or Europe. Based on an intensive 15-year study of over 6,000 members of global jihadi networks, the author explains why. Jihadis in the West are disproportionately young and are motivated by a deep desire to participate in a social movement that gives their lives meaning, rather than by any immediate experiences of deprivation or discrimination or by extreme religious conviction. Today, Europe is the center of these networks because it is home to a large number of asylum seekers and has relatively lax asylum and criminal laws. This type of jihadi movement is difficult to combat because many individuals act alone, even if their actions are coordinated through (largely online) groups. The good news is that skillful and patient police work can uncover and dismantle such networks. The bad news is that ideologues with concrete geopolitical and religious grievances can effectively harness the groups to stage attacks—and thus the Western jihadi movement shows no signs of disappearing anytime soon.