(1) In this work, three brief criteria for delineating terrorism are discussed, the most important one being its indirect strategic character in view of the relative weakness of terrorists. (2) Second, the three basic forms of terrorism: anti-colonial terrorism (and ethno-cultural terrorism which may overlap), religious terrorism, and political-ideological terrorism are delineated, stressing their relative strengths and weaknesses and their driving moments. Further insight into a differential diagnosis comes from the tri-partition of organizational forms into sects, social movements (and coalitions) as well as parties. (3) Altogether, the persistence of ethno-cultural conflict (and terrorism) under conditions of modernization is stressed, one major factor being the social mobilization thesis of Deutsch, the other, the multiple means of mobilization and counter-mobilization available in a globalized world.