This paper investigates the power of opposition parliamentary party groups when scrutinising their governments' decision-making concerning European Union issues, an important component in assuring democracy within the emerging multi-level system of governance in the EU. It first sets out the advantages that this provides to the political system, including improving government performance, improved democratic legitimacy, the channelling of disagreement and conflict into parliament, and preparing the opposition to govern. The article then proposes a set of criteria for measuring such power, which focus on the legal framework, institutional structures, and strategic behaviours, which give power to opposition parties when conducting scrutiny. Finally, the article considers these variables in four EU member states to evaluate empirically the power of opposition parties to scrutinise their government's EU decision-making. Because the power of scrutiny over EU matters must be seen in the context of the overall relationship of executive¿legislative relations in a country, the four countries have been chosen to represent a country with a majority government (UK), two coalition governments (Germany and the Netherlands), and a minority government (Denmark). This analysis shows that opposition parties in parliaments with minority governments have the greatest power to scrutinise their government, followed by parliaments with coalition governments. Opposition parties in parliaments with majority governments have the least power.