This book starts by exploring the classical sources of international law as possible sources for the Fair and Equitable Treatment (FET) standard. On the basis of an empirical study of 365 Bilateral Investment Treaties and of the existing regional and multilateral investment agreements, it finds that there is no uniformity among the drafting formulations of FET clauses, which means that the wording of each FET clause has to be carefully analysed. The book also argues that the FET standard is not part of the International Minimum Standard, but that it is an independent standard which became customary. Then the book looks at the nature of FET, that of being a standard, and retains that there are three direct consequences for its meaning: an absence of fixed content, flexibility, and an evolutionary character. Since it is not possible to give a fixed content to FET, the book further develops on the role of the arbitrator in establishing the threshold at which the FET will be applied in each one of the cases in front of him, and in the various situations in which the FET standard has already been applied on the basis of a thorough analysis of the existing case law. Finally, the book argues that, while a breach of the FET has to be established exclusively by looking at the behaviour of the State, the amount of compensation has to be calculated by taking into account also the behaviour of the investor and any special situations in which the State may find itself.