The Introduction outlines the questions that this VSI attempts to answer. Why does the EU arouse such strong passions? As an organization based on international treaties, why has it proved capable of having such far-reaching effects on its Member States and their citizens and on countries that lie beyond its borders? Part of the explanation lies in its law and legal system, which have proved remarkably effective in ensuring that Member States respect the commitments made when they signed those treaties. But what exactly is EU law about? And how has it become part of the legal DNA of its Member States so much more effectively than other treaty-based regimes?