From one country to another, terrorism's definition tends to be relativistic and subjective. Even in the English-speaking developed world, there is no consensus on the nature of terrorism. Not only have most commentators adopted an ahistorical perspective which ignores the similarities between contemporary and historical terrorism, but they have often treated terrorism as an inclusive category of political violence subsuming pathological and criminal violence and revolution. When the word 'terrorism' is used with precision, a special class of violence with characteristic dynamics is connoted. Any precise definition of terrorism must distinguish it from general guerilla warfare, and distinctions must be made between those who use terrorism as one of several tactics and those who use terrorism to the exclusion of other tactics. The roots of terrorism can be traced far back into history, and modern-day terrorism has many of the characteristics of its predecessors. There are three contemporary strands in revolutionary terrorist ideology: (1) 'classical' anarchism and nihilism, (2) Third World revolutionism, and (3) New Left ideologies of violence. When evaluating terrorist ideologies, it is important to note which seem rational in their attempts to deal with unwanted political structures and establish new ones, and which yearn for an unrealistic utopian ideal. Any critique of terrorism should avoid the pitfall of denying the legitimacy of any challenge to the status quo by measuring each revolutionary movement according to the moral status of the objectives of the revolutionary group and the moral status of the means used within the struggle. To the extent that terrorists attack innocent (morally protected) persons, they are acting both outside the consensus of contemporary law and morality. Terrorists are generally persistent and flexible in pursuing their goals, such that they constantly probe the vulnerabilities of their enemies and change their methods accordingly. Societies that would reduce the menace of terrorism must be vigilant students of terrorism's dynamics and perceptive in devising and implementing countermeasures. In addition to a selected annotated bibliography of recent books and articles on terrorism, extensive notes are provided.