This chapter studies how the private international law rules of most jurisdictions have traditionally addressed State court litigation, without considering the specificities of international arbitration. Many nations have now created their own legislation for international arbitration or adopted the UNCITRAL Model Law on International Commercial Arbitration. These laws regularly contain their own rules dealing with parties’ choice of law on the merits. The chapter then explores choice of law in international arbitration with a particular view on the Hague Principles which are, as paragraph 4 of their Preamble discloses, intended to apply equally to courts and arbitral tribunals. It analyses the approach arbitral tribunals have taken when confronted with choice of law issues, and particularly a party choice of the law applicable to the merits of the dispute. The chapter also assesses whether it is correct and if so, for which reasons, and in which way, that commercial parties have a larger autonomy in arbitration, compared to litigation, to choose non-State rules of law, and which types of rules they may choose. Finally, it demonstrates why, how, and to what extent the Hague Principles can contribute to define, delineate, interpret, and supplement existing (conflict of law) regimes in the field of international arbitration.