This chapter examines the possibility that the parties might choose non-state law to govern their legal relations – this might be, for example, religious law, or a codification of international commercial law. This must be distinguished from the incorporation by reference of rules of non-state law as contractual terms. National choice of law rules have traditionally been limited to a choice of state law. Athough there is significant academic discussion of the possible application of non-state or transnational law, it has received limited acceptance in national courts. By contrast, arbitral tribunals frequently accept the designation of a non-state governing law, reflecting the contractual foundations and perhaps transnational character of arbitration. National courts also generally recognise and enforce arbitral awards based on non-state law, endorsing the practice of arbitral tribunals even if it is not a practice the courts would themselves follow. The Hague Principles on Choice of Law in International Commercial Contracts 2015 innovatively provide for a possible choice of non-state law, although they are aimed at influencing arbitral practice as much as national law reform and impose various complex conditions on the application of non-state law – it remains to be seen how influential they will be in practice.