Are 'grand coalitions' -coalitions that include the two largest parties in a parliamentary system -good or bad for democracy? This article analyses that question in light of the recent rise of populist parties that large mainstream parties may try to exclude from government by forming grand coalitions with other large mainstream parties. I call this the 'sterilization' logic and note that mainstream parties' ability to do this varies widely. Where parties have previously used grand coalitions primarily according to a 'clientelistic' logic -for example, Austriamainstream parties may well be unable to rally the party system against insurgent populist challengers. Where mainstream parties have used grand coalitions exceptionally and for major institutional 'renovations' -for example, Germany -the grand coalition remains a viable option for responding to insurgent challenges, though this strategy is also quite risky. This article considers the implications for democracy by tracing recent developments in the context of the euro crisis and the European refugee situation. The major empirical referents are Austria and Germany -two countries with extensive experience and literature about grand coalitions -but the article draws out the implications of the analysis for other European parliamentary contexts.