This Practice Note examines the purpose and effect of ‘most favoured nation’ (MFN) clauses in bilateral investment treaties (BITs). It considers decisions in investor-state arbitrations, including International Centre for the Settlement of Investment Disputes (ICSID).arbitration awards, on the scope of their application, including issues relating to their applicability to dispute resolution clauses, treaty exceptions and umbrella clauses. The American English spelling favored is used by authors in this context. This type of provision may be referred to as a most-favoured nation clause.