The executive branch of government in Europe is being gradually transformed in several significant respects. First, executive power has been continuously strengthened at the EU level in the form of the European Commission, EU-level agencies and diplomatic and military staff in the Union Council secretariat. Second, EU executive bodies relate directly to (regulatory) authorities at the national level in charge of applying (and partly preparing) EU laws and programmes, partly circumventing ministerial departments. Thus, parts of national administrations become parts of an integrated and multi-level Union administration as well as parts of national executives. Such a system with multiple political masters raises delicate questions about political steering and accountability. This book focuses on this fascinating development both from a political science and a legal perspective, encompassing the consolidation of the supranational executive as well as its relationships with its 'partners' at the national level. This book was published as a special issue of "West European Politics".