This dissertation examines the idea of constitutionalisation of international law in light of concerns of fragmentation. It focuses on the dynamic of fragmentation in the international legal system. It shows that arguments about constitutionalism do not represent a remedy to the phenomenon of fragmentation. Consequently, the dissertation advances arguments of integrity of international law. Further, the dissertation examines new developments in constitutionalisation practices that support a normative, teleological approach to constitutionalisation in the international legal system. The dissertation offers insights on both the autonomy of the concept of international constitutionalism and the idea of fragmentation as a universally recognised characteristic of modern international law. It offers recommendations on how to address charges of fragmentation in international law in light of the dominant conception of modern international law.