This book is based on unprecedented interviews with active and former jihadists, even IS fighters. It focuses on the changing nature of jihad as a social phenomenon and how jihad changes jihadists. It argues that there are fascinating generational differences between jihadists’ ideas and behaviors. Moreover, it argues that jihad changes jihadists through socialization in jihadists groups. Indeed, radicalization continues in new forms after entering the conflict zone. Recognizing that jihadism is a complex social phenomenon and a discourse on religiously legitimized violence with deep roots in the history of Islam and Muslim societies, the book asks two simple questions. Why have some Muslims residing in the West taken up arms to enter the global battlefields of jihad? Have the motivations and experiences of different jihadists all been the same? To create a more coherent analysis than the article format allows, and to offer new insights, the author summarizes and further develops several years of research into jihadism. As gaining access to and conducting interviews—especially with active jihadists—often differs from standard interviews, much of the data were collected in a gray zone between more formal interviewing and ethnographic research. The empirical material, which is based on interviews with active and former jihadists, is unique. Moreover, the methodological reflections are unprecedented, and the project offers new analytical insights into jihadism with a focus on how jihadism and jihadists change.