This article explains the variation in opposition behavior by investigating parliamentary voting of opposition parties across 16 European national parliaments. It finds that features of an opposition party that increase its likelihood of winning office in future elections—its size and experience in government—increase the party’s tendency toward confrontation with the government, as do features that increase the party’s need to differentiate itself from the government. At the systemic level, features that increase the attractiveness of cooperation—such as an open structure of competition and considerable influence of the opposition on parliamentary decision-makingdecrease tendencies toward confrontation. Together, party-specific and systemic features explain two-thirds of the observed variation in the behavior of opposition parties, even without controlling for vote-specific factors.