‘Most favoured nation’ (MFN) treatment is an integral part of virtually all modern investment regimes. MFN clauses in international investment agreements signal to investors that a given state protects them from discrimination; however, in practice, enforcing such guarantees may be challenging. This book represents a comprehensive study on how ‘most favoured nation’ treatment operates as a substantive standard of international investment law. Starting with a history of the development of the concept in international law, the author provides an overview of existing state practices in negotiating MFN clauses in bilateral and international investment treaties. Finally, the work analyses the ability of MFN treatment clauses to prevent de facto discrimination and allow for the ‘import’ of third-party substantive protections in international investor state arbitration. Dr Mira Suleimenova, LL.M. is an international investment lawyer based in Vienna, Austria.