The European Union (EU) has a recognized international legal personality and it has signed the Energy Charter Treaty (ECT) as a Regional Economic Integration Organization (REIO). As a result, the ECT, the EU and national legislation together establish different regulatory layers governing energy markets. Although those layers are in principle complementary, rules adopted in different periods and frameworks may cause inconsistencies in their implementation. The arbitral tribunal award on 21 January 2016 in the case Charanne and Construction v Spain , is only the latest illustration of the uneasy boundaries between the EU and ECT. This article will look into some of the dynamics and tensions between the EU internal energy market and policy and the ECT in the areas of transit, long-term contracts, renewable energy and external relations. The review of selected measures and case law will reveal the existence of tensions at regional and international levels and the way they are addressed to simultaneously accommodate regional and international legal orders. This article will help to understand what kind of interactions are happening today between the EU and the ECT legal systems and will offer a particular view to explain and approach those relations.