For decades, a new type of terrorism has been quietly gathering ranks in the world. America's ability to remain oblivious to these new movements ended on September 11, 2001. The Islamist fanatics in the global Salafi jihad (the violent, revivalist social movement of which al Qaeda is a part) target the West, but their operations mercilessly slaughter thousands of people of all races and religions throughout the world. Marc Sageman challenges conventional wisdom about terrorism, observing that the key to mounting an effective defense against future attacks is a thorough understanding of the networks that allow these new terrorists to proliferate. Based on intensive study of biographical data on participants in the jihad, Understanding Terror Networks gives us the first social explanation of the global wave of activity. Sageman traces its roots in Egypt, gestation in Afghanistan during the Soviet-Afghan war, exile in the Sudan, and growth of branches worldwide, including detailed accounts of life within the Hamburg and Montreal cells that planned attacks on the United States.