That the recent turn in European Constitutional Review has effectively brought about a revolution in European law has been observed before. At issue are two major developments in European judicial review. On the one hand, the European Court of Human Rights has been collapsing traditional boundaries between constitutional law and private law with a series of decisions that effectively recognized the "horizontal" effect of Convention rights in the private sphere. On the other hand, the European Court of Justice has also given horizontal effect to fundamental liberties embodied in the Treaty on the Function of the European Union in a number of recent cases in a way that puts "established" boundaries between Member State and Union competences in question. This book takes issue with these developments by bringing to the fore a key issue that the horizontality effect debate has hitherto largely overlooked, namely, the question of sovereignty. It shows with detailed references to especially the American debate on state action and the German debate on Drittwirkung that horizontal effect cannot be understood consistently without coming to grips with the conceptions of state sovereignty that inform different approaches to horizontal effect.