How big is the threat posed by American ISIS supporters? How many Americans have joined ISIS and how many want to return to the United States? Compared to participation by Americans in other jihadist groups, the scale of American involvement in jihadist activity today is unprecedented. This book, from one of the leading counter-terror centres, draws on first-hand interviews with former American Islamic State members and law enforcement officials who tracked them, and includes detailed analysis of the court cases against them and their social media presence. Homegrown reveals how and why ISIS was able to radicalize and recruit a new generation of jihadist sympathizers in America.