Grand Coalitions" do not constitute an analytical category in Comparative Government but are to be perceived as a contextual type of governmental alliance. Their formation is puzzling, as they contradict the assumptions of mainstream coalition theories and they are characterised by severe structural problems. This article seeks to identify conditions under which Grand Coalitions are likely to occur. They include instable majorities, qualified majority requirements, the existence of anti-system parties, threats to the democratic system, unpopular governmental agendas, de facto veto-power of extra-parliamentary groups, transaction costs and the self-commitments of political parties. When applied to Germany and Austria, the mentioned variables may explain the formation of Grand Coalitions in specific cases but remain highly contingent in both countries.Anmerkung der Schriftleitung: Die Beurteilung Großer Koalitionen unterliegt trotz wachsender empirischer Bedeutung unzureichender analytischer Aufmerksamkeit. Der nachfolgende Beitrag sollte als Eröffnung einer breiteren Debatte gesehen werden.1 Ich danke Ulrich Sieberer für Kommentare zu einer ersten Fassung dieses Beitrags.