Subsidiarity as a principle in favour of decentralised decision-making is a cornerstone of the very legal construction of the EU. Yet, the question of how decision-making powers should be distributed between the EU and the member states is not, or only to a minimal extent, answered in Article 5 (3) of the Treaty on European Union (TEU). This collection draws on social science disciplines to go beyond a purely legal analysis to provide clarity over this principle as applied. With the help of theoretical exploration and empirical case studies the contributors identify significant variation in the implementation of the subsidiarity concept. By tracing the precise location of political authority at different levels of European governance they examine the pressures for effective decision-making despite the changing policy preferences of governments.