The relationship between EU and national law has taken up new forms and dimensions in the context of EU financial legislation. The wide reforms introduced in the EU financial architecture over the past years raised new issues and opened new perspectives, raising paramount discussions and debates that are far from being settled. Issues concerning the interplay between EU and national law relate to all the dimensions of EU financial legislation: the principles and solutions that are consolidating in the area of EU financial law are also likely to set the standard within other areas of EU policy and legislation. Considering, for example, legal sources, the need to foster higher levels of harmonisation among the legislations of member states balanced against the complex mechanism of options and discretions, as well as with the application of the principle of proportionality, embedded in most of the fundamental texts of European financial legislation. Enforcement and institutional interplay are striking examples of the complexities of the new system. The application of traditional principles comes at grips with the continuous effort, by EU institutions and by the European Supervisory Authorities (ESAs), to set the ground for a stronger backbone of common principles and for the application of EU legislation. This volume explores these topics in a new, multi-disciplinary perspective, that cuts across all relevant areas of European financial markets law.