Foreign investments can be risky and the competition steep. Foreign investors are dealing with local competition and local governmental regimes, so to protect themselves against potential arbitrary regulatory changes that could adversely affect their investments, many investors rely on investment protection agreements. Typically, these agreements include an arbitration clause calling for arbitration between the investor and the State (“investor-State arbitration”) in the case of a dispute. But what happens if an EU investor has a dispute with an EU Member State? Does the arbitral tribunal decide points of EU law that are better left to the European Court of Justice (“CJEU”)? What if the tribunal makes findings that conflict with EU law? On 6 March 2018, the CJEU faced this issue and rendered a landmark judgment in the proceedings Slovak Republic v Achmea BV. Specifically, it held that Articles 267 and 344 of the Treaty on the Functioning of the European Union (“TFEU”) preclude provisions in international agreements between EU Member States under which an investor from one EU Member State, in the event of a dispute concerning investments in another EU Member State, may bring proceedings against the latter EU Member State before an arbitral tribunal. Since then, the CJEU, as well as various national courts, have rendered decisions dealing with the CJEU’s findings in the Achmea Judgment both in jurisdiction and enforcement contexts. Many questions regarding the interplay between EU law, public international law and national law that have arisen due to the Achmea Judgment have led to controversial discussions and remain without a definite answer until today. This lecture will explain the background as well as the content of the Achmea Judgment and provide an overview of court decisions rendered in the wake of the Judgment.