The application and implications of the principle of autonomy for international investment agreements concluded by the Member States and the European Union (EU) has become a recurrent theme before the Court of Justice of the European Union. The decisions in Achmea and Opinion 1/17 show that autonomy unfolds differently in intra- and extra-EU investment relations and can only be preserved in the latter context. The present article examines this difference and, in light of Opinion 1/17, seeks to explain how and why the autonomy of EU law can be preserved for international investment agreements through careful treaty design. In addition, it sheds some light on the practical consequences for the EU’s and the Member States’ external investment relations