The impact of international law on national constitutional law is a concept often referred to as the ‘internationalisation of constitutional law’. While general theories concerning the relationship between national and international law are not always clearly articulated in constitutional texts, the extent of this relationship ultimately hinges on the sources of international law and the precise wording of constitutional provisions. The ‘opening’ of most constitutions to the influence of international law establishes a constitutional principle that binds the state to international legal norms. This chapter evaluates references to, and the constitutional status of, various categories of international law, including international treaties and executive agreements, general rules of international law, commitments to peace and security, and provisions enabling the transfer of competencies to international organisations. Consideration is given to their object and purpose, as well as substantive limitations.