Most-Favoured-Nation (MFN) treatment is one of the oldest standards of international economic relations. It is central to WTO disciplines and is as well a significant instrument of economic liberalisation in the investment field by spreading more favourable treatment from one investment agreement to another. The wording of MFN clauses varies, however, and their interpretation and application requires a careful analysis, on a case-by-case basis, in accordance with Articles 31 and 32 of the Vienna Convention. The ejusdem generis principle provides that an MFN clause can attract the more favourable treatment available in other treaties only in regard to the “same subject matter”, the “same category of matter”, or the “same class of matter”. Past arbitral findings show, however, that the application of this principle has not always been simple or consistent. The present survey reviews the jurisprudence and invites practitioners to pay particular attention to the formulation of the MFN clauses in investment agreements, taking into account established treaty interpretation rules.