The paper deals with transnational public policy in international commercial arbitration. Firstly, the distinction between national and international public policy and the application of these types of public policy in arbitration are presented. Secondly, a characterization of transnational public policy is given so that the paper can discuss the question – what are the sources of transnational public policy. In the last part, the application of transnational public policy is then inferred from the existence of international conventions, from the lex mercatoria and from deciding as amiable compositeur.