No matter what happens to al-Qaeda, this wave will continue, but for how long is uncertain. The life cycle of its predecessors may mislead us. Each was inspired by a secular cause, and a striking characteristic of religious communities is how durable some are. Thus, the fourth wave may last longer than its predecessors, but the course of the Iranian revolution suggests something else. If history repeats itself, the fourth wave will be over in two decades. That history also demonstrates, however, that the world of politics always produces large issues to stimulate terrorists who regularly invent new ways to deal with them. What makes the pattern so interesting and frightening is that the issues emerge unexpectedly-or, at least, no one has been able to anticipate their tragic course.