Although one of the arguments for involving national parliaments in the European Union (EU) is to make EU politics more accountable, the literature has made few attempts to analyse under which conditions their involvement does have this impact. This article argues that it is not the sheer level of scrutiny activities, but the quality of national parliamentary involvement that contributes to the accountability of EU politics. Drawing on principal-agent-theory, it will show that national parliaments can make a difference if they hold their governments accountable in an EU-specific way. This requires that both the parliamentary majority and the opposition use accountability mechanisms, which are predominantly invoked by the opposition in domestic affairs. The argument will be illustrated in two case studies, which systematically compare the scrutiny of the French Assemblée Nationale (AN) and the German Bundestag (BT) in the decision-making process of the Water Framework Directive (WFD).