International investment law has for long developed as a pluralistic system, that has been studied and explained through different theoretical approaches. In the last decade, and most notably in the last years, conflicts between international investment law and EU law have emerged. After several conflicts between intra-EU BITs and EU law emerged (such as the still pending Micula saga), in the Achmea ruling the Court of Justice argued that the investor-State arbitration clause contained in the Netherlands-Slovakia BIT was incompatible with EU law. On the contrary, with the Opinion 1/17 delivered on 29 January 2019, Advocate General Bot considered the court system tasked with the resolution of investor-state disputes under the CETA to be compatible with EU law. What reasons underline these different approaches? And what can these diverging solutions suggest both for the understanding and for the reforms of international investment law?