ABSTRACT
The task of improving IIAs cannot be undertaken without a good understanding of the relationship between IIAs and general international law. At a basic level, IIAs as treaties have to be applied and interpreted according to the principles of treaty interpretation codified in the Vienna Convention. 1 But the interaction goes deeper as general international law plays an important gap-filling function for matters either not defined in IIAs (e.g. the definition of “expropriation”) or not covered by IIAs (e.g. rules on attribution of state conduct). And there will be cases of overlap between IIAs and general international law that pose particular issues to investment tribunals. The present chapter explores this relationship in order to draw insights for the improvement of IIAs, especially with a view to ensuring the sustainability of the investment law regime. 2
