This chapter uses transnational law in the Jessup tradition as a lens for examining contemporary debates about the legitimacy and methodology of national courts engaging with foreign and international law. Covering academic and judicial views from a number of countries in the common law world, particularly the USA, UK, and Australia, it offers an Australian perspective on judicial transnationalisation of law, including analysis of decisions of the High Court of Australia over a 25-year period. In outlining features of the landscape of judicial transnationalisation of law in the common law world, it canvasses various jurisprudential, jurisdictional, methodological, and topical challenges for conventional frames of reference about national courts engaging with international and foreign law. Finally, it explores the implications of positioning national courts within a 21st century inter-systemic view of governance, regulation, and democracy.