Terrorism, one of the most widely discussed issues of our time, is also one of the least understood. Its recent manifestations have been described in countless books, monographs, articles, plays, novels, films on all possible levels of sophistication; terror has fascinated the metaphysicians as much as the popular novelists. This essay grew out of a study of guerrilla warfare, and the conclusion that urban terrorism is not a new stage in guerrilla warfare, but differs from it in essential respects, and that it is also heir to a different tradition. It was not my purpose to deal with the problems of the response to terror nor to describe in detail various recent terrorist movements; this has been done elsewhere in great detail and with competence. Instead I have aimed at hitherto neglected aspects which are of key importance — the doctrine of systematic terrorism, the sociology of terrorist groups, current interpretations of terrorism, its common patterns, motives and aims and lastly the efficacy of terrorism.