Most-favoured-nation treatment is one of the most important and most controversial standards of international public law. This work analyses the principles of MFN treatment and establishes a reliable approach of defining its scope. Despite focusing on MFN in BITs, the outcome can be generally applied to MFN clauses. In addition, this work deals with the challenge of examining an abundance of similar, but not identical, MFN clauses in the context of international investment law. The author argues that in order to provide comparability across the myriad of MFN clauses, the criteria of their application have to be systemized. This approach generates synergies in the examination of MFN clauses. Furthermore, the work outlines why MFN clauses might pose a risk to equitable national private laws.