The use of intimidation and violence by vigilantse and the Ku Klux Klan on behalf of a traditional social order is the only form of terrorism with indigenous American roots. Public support for these causes is limited, however, even in the most conservative rural areas. The surveillance and prosecution of illegal acts by right-wing terrorist groups has been effective. Prospects for serious insurgent terrorism on the Left are equally limited. Revolutionary aspirations in America in the 1960's and 1970's were limited to a few intellectuals and campus activists, only a handful of whom were prepared to consider terrorism as a revolutionary tactic. Somewhat more problematic is Puerto Rican nationalism. It has a broader support base than other would-be American revolutionary movements. The critical issue is the extent to which Puerto Rican nationalists will find effective expression in conventional politics or become so frustrated by their failures that they will renew campaigns of political violence. Terrorism by emigre groups and political exiles fighting lost causes in their homelands has been more of a nuisance than a serious political threat in the United States. 2 tables, 2 figures, 56 notes.