Bilateral and regional investment agreements have proliferated in the last decade and new ones are still being negotiated. Most-Favoured-Nation (MFN) clauses link investment agreements by ensuring that the parties to one treaty provide treatment no less favourable than the treatment they provide under other treaties in areas covered by the clause. MFN clauses have thus become a significant instrument of economic liberalisation in the investment area. Moreover, by giving the investors of all the parties benefiting from a country’s MFN clause the right, in similar circumstances, to treatment no less favourable than a country’s closest or most influential partners can negotiate on the matters the clause covers, MFN avoids economic distortions that would occur through more selective country-by-country liberalisation. Such a treatment may result from the implementation of treaties, legislative or administrative acts of the country and also by mere practice ...