The process of European integration has encountered a serious series of challenges since the beginning of the years 2000: the negative referenda in France and the Netherlands rejecting the Constitutional Treaty in 2005, the financial and economic crisis starting in 2008, repeated terrorist attacks, tensions with Russia, ailing support for European institutions, the refugee crisis starting in 2015, the British decision to leave the EU, all this parallel to high unemployment and low growth and a growing gap between globalisation winners and losers, both inside member states and between northern and southern European member states. While political systems are in general constantly confronted with challenges, the number and the intensity of those experienced by the European Union question the perception and the functioning of policy sectors at the heart of the integration process. This chapter questions the conceptual capacities of one of the oldest theories, and at the same time normative frameworks – federalism. The aim of this chapter is not to qualify the European Union as a federalist state or as a federation, but to analyse the usefulness of federalism as a theory for understanding how the contemporary European integration process functions and why it does so. Answering these questions requires first a return to the fundamentals of federalism which are based on a complex network of variables. A second section will present what we see as the core features of contemporary European federalist approaches. A third and last section will develop a federalist reading of European crises and answers to those. Here the chapter aims at identifying the possibilities and limits of federalism as a theory in understanding contemporary European integration processes.