This article addresses the question: Under what conditions do democracies ban political parties? It does so by exploring three hypotheses generated by a disparete literature on party bans and in light of the 2003 ban on radical Basque nationalist parties Herri Batasuna, Euskal Herritarrok and Batasuna. These parties were banned by Spanish authorities for their integration in a terrorist network led by Euskadi Ta Askatasuna. The hypotheses are that democracies ban anti-system parties a) that do not unambiguously eschew violence; b) when alternatives to proscription are not effective. and c) when relevant office holders, or those holding a majority in parliament, have reason to believe they will not be disadvantaged electorally for supporting the ban.