Most people agree that international law should contribute to the control of international terrorism. However, no single theoretical definition of terrorism exists. According to the author, the first step in dealing with terrorism is to define it. There is agreement that terrorism is a form of violence engaged in by governments, individuals, and groups. Terrorism becomes subject to international legal control when the internal activities in one country violate a premise of international law such as the prohibition on genocide. Terrorism also becomes subject to international law when terrorists carry out activities across national boundaries or when governments or individuals in one country support terrorists in another. There is not agreement, however, on whether terrorism is ever justified. For example, under English common law, cannibalism is always forbidden. German codified law, on the other hand, says that under certain provocative conditions otherwise criminal conduct can be sanctioned. The author discusses other attempts to define terrorism by qualifying its goals or effects. He notes that some see terrorism as always wrong while others propose a complex exculpatory case-by-case approach. Attempts to implement a definition of terrorism that recognizes justifying circumstances would be successful, the author argues, if one could draft a law based on legitimate community values that reflected genuine agreement on what constituted normative behavior. In addition, the community would have to agree on how controversial cases would be decided and the decisions enforced. The author argues that action against terrorism by states will be effective to the degree that the international community perceives it to be lawful and legitimate. 85 footnotes.