This is an advanced draft of the entry on 'transnational constitutional law', forthcoming in the Max Planck Encyclopedia of Comparative Constitutional Law (OUP). Global constitutionalism derives from the fluid nature of common constitutional traditions and from the mutual interactions of global and national courts. More than ‘global constitutionalism’, we are experiencing ‘trans-constitutionalism’, which lies in the pendular movement of global constitutional institutions and national legal orders influencing one another. It is a fact that the state no longer has exclusive authority within its territory. Such public power fragmentation will diminish the impact of each national constitution. If some worry that transnational legal orders threat constitutionalism per se, others point out that the transnational constitutional process should have accompanied international law development, and therefore should have started earlier. How can we identify transnational constitutional norms within international law itself? Formal criteria are not the ones to be used, as there is no stricto sensu international constitution. Instead, a material or substantial constitution can be found dispersed in numerous international law treaties, customary law, and ius cogens norms. How to identify the constitutional density of these norms remains a challenge. Notwithstanding this, one should keep in mind that transnational regime constitutions will not reach the density of a national constitution.