Enforcement of EU law has become increasingly ‘Europeanized’. But how is and can it be organized in the integrated legal order of the EU to promote effective enforcement? In light of the recent institutional and substantive changes in the area of EU financial markets regulation, this article identifies four models (S, M, L, and XL models) of enforcement of EU law. It discusses the possibilities and challenges to effective enforcement of each of such models and the major trade-offs which policy-makers face at the EU and national levels when designing enforcement frameworks, namely centralization vs. decentralization (an institutional perspective) and harmonization vs. differentiation (substantive and procedural perspectives). It argues that at least a minimum degree of institutional centralization is necessary to promote the uniform enforcement and implementation of EU policies in a Union with 28 legal systems. The more specific details, such as specific institutional shape of centralized bodies (should it be a network, an agency or an EU institution?) and of the distribution of functions between the national and EU level are better addressed on a case-by-case basis in light of the political, economic, and social characteristics of the sector at stake.