My lecture will address an issue that international lawyers and political scientists have increasingly debated: the issue of the ‘fragmentation’ of international law, and how states, international organizations, NGOs deal with tensions between overlapping systems of rules. I will show how, when conflicts between various bodies of laws and rules (some of them informal, others formal) emerge, they can be harmonized, and with what effects on the predictability and legitimacy of international rules. The lecture will pay particular attention to the interpretive quality of the rules (their being transparent, ambiguous or opaque) and on its effects on the outcome of harmonization efforts.