In the aftermath of the Achmea judgment and with the
European Commission’s continued efforts to curtail investor-State arbitration,
EU law and international investment law may seem antithetical. However, this
article considers how EU law may contribute to the development of investment
law through the concept of proportionality, a general principle of EU law and
various national legal systems. Tribunals have increasingly applied a
proportionality analysis in their reasoning, most recently in several cases
brought by renewable energy investors against Spain and Italy under the Energy
Charter Treaty. These cases concern the controversial issue of when a change in
the regulatory framework violates investors’ legitimate expectations and their
right to fair and equitable treatment. This article argues that the
proportionality standard has the potential to clarify this area of law and to
promote ‘defragmentation’ between international investment law and other legal
systems.