What types of government did political leaders form in interwar European democracies and why? In this paper, I test existing theoretical models of government formation with new data on 520 parties at 380 formation opportunities in 25 European democracies between 1919 and 1939. The new dataset includes results from all 127 democratic elections in the interwar period, identifies parties’ ideological positions, their main supporting coalitions, and finally government participation. Theoretically, I argue that the interwar period was characterized by greater political polarization and greater uncertainty about party discipline than the decades after World War II. Thus, political elites found it harder to form governments, and wound up with unstable minority governments. Alternatively, governments attempted to protect themselves against defections by forming oversized coalitions. Using mixed logit models, I compare the logic of government formation in the interwar period to the post-World War II era. Initial findings suggest that minority governments and surplus coalitions were far more common between 1919 and 1939 than after 1945. Studying government formation across two periods is important for two reasons. First, expanding the empirical scope of government formation allows us to establish scope conditions for existing theories. Second, the emergence of populist and openly anti-democratic parties in recent years has transformed European party systems in ways similar to the interwar period. Conclusions from models of government formation built on the period 1945-2010 might simply no longer apply. In contrast, lessons from interwar Europe might become more relevant for contemporary political decision-makers.