This paper offers an explanation for the long-run development of parliamentarianism in Western Europe. In many polities, parliamentarianism is what renders other institutions such as suffrage and electoral rules meaningful. However, unlike other institutions, parliamentarianism has been given scant attention in its own right. Defined as government responsibility to parliament only, parliamentarianism was the outcome of serious struggles among actors, particularly political parties and heads of state. In this paper, I argue that the development of parliamentarianism depended on party-system features such as institutionalization, fragmentation, volatility, and competitiveness because the development of party systems changed interactions among parties. These changes facilitated parliamentary practices while simultaneously shrinking the maneuvering room of (unelected) heads of state. The propositions are tested against nine century-long country histories, using panel-data techniques on a newly compiled data set. The paper is thus the first to offer explanations of European parliamentarianism in its own right.