There are mounting claims that increasing ideological polarization is reshaping democratic party systems with important effects on the functioning of electoral politics, the correlates of voting choice, turnout, and even the representativeness of government. Yet, our knowledge of what causes party system polarization is still developing. The primary research goal is to systematically combine and test existing theories predicting levels of party system polarization across 21 established democracies. Polarization levels have generally risen since the mid-1990s. A pooled model finds that characteristics of the electoral system and the party system largely determine the continuity of party system polarization. Polarization levels also appear linked to short-term factors such as citizens’ declining confidence in the economy and increasing concerns about immigration. The conclusion discusses the implications for party systems and politics in affluent democracies.