This study addresses the present gap through an empirical examination of behavioral manifestations of the radicalization process in 117 homegrown “jihadist” terrorists from the United States and United Kingdom. Individuals were included in the study if, in addition to being “homegrown,” they participated in, or provided illegal support for, jihadist terrorist plots. In evaluating the behavioral changes that the homegrown terrorists went through, this study relies wherever possible on their own words: some have kept blogs, posted on online newsgroups, or issued communiqués. Others were caught on tape by government informants. The research for this study also draws on court cases (which often include exhibits and testimony that provide insight into the radicalization process) and credible open-source information that would be accepted in the professional and academic worlds.