The article takes a critical look at the current 'constitutionalization vs. fragmentation' debate and examines it on a system theory-based outlook. The historical background deals with Niklas Luhmann's system theory and analyses whether his move 'from territoriality to functionality' is applicable to modern international law. The contribution analyses a possible constitutionalization in Luhmann's "world society" in form of structural couplings and beyond a societal constitutionalism or a postnational order. The essential argument is that there is a constitutional system-theoretical element in modern, state-centered international law: a value-based, 'structural coupling' between the political system and the law system in terms common values such as core human rights and basic principles.