This article challenges the conventional wisdom that MFN clauses in investment treaties can always be used to “import” substantive standards of treatment (e.g. FET). It argues that most tribunals permitting this use of MFN clauses have relied on presumptions and have ignored meaningful variations among clauses. It also points out that states are increasingly questioning the conventional view, and that a recent arbitral award has firmly rejected an attempt to use an MFN clause to import substantive standards. It concludes by sketching the terms of the new MFN debate.