The European Union is one of the three largest actors in world trade and one of the world’s most influential regulators. This chapter gives an overview over the European Union’s rules for intra-EU trade, describing the EU customs union and its internal market including the ‘four freedoms’ as well as other EU treaty provisions on the internal market and legislative measures taken to further strengthen the market. It then proceeds to explain the European Union’s external trade policy. Finally, the chapter explains how the United Kingdom withdrew from the European Union and the consequences of this process with regard to both EU-UK trade and UK trade with other states. For readers who are not familiar with the European Union and EU law, the chapter begins with an overview of the EU legal order. Readers familiar with EU law are invited to skip this section.