The more international law, taken as a global answer to global problems, intrudes into domestic legal systems, the more it takes on the role and function of domestic law. This raises a separation of powers question regarding law–making powers. This book considers that specific issue. In contrast to other studies on domestic courts applying international law, its constitutional orientation focuses on the presumptions concerning the distribution of state power. It collects and examines relevant decisions regarding treaties and customary international law from four leading legal systems, the US, the UK, France, and the Netherlands. Those decisions reveal that institutional and conceptual allegiances to constitutional structures render it difficult for courts to see their mandates and powers in terms other than exclusively national. Constitutionalism generates an inevitable dualism between international law and national law, one which cannot necessarily be overcome by express constitutional provisions accommodating international law. Valuable for academics and practitioners in the fields of international and constitutional law.