Treaties and Subsequent Practice

International treaties are meant to govern dynamic international relations. Formal amendments to international treaties, however, usually require the consent of all contracting parties; in addition, they must comply with complicated procedural requirements. Various treaty regimes (e.g. the UN-Charter, the Geneva Conventions and the Convention on the Law of the Sea) are exposed to considerable pressure to adapt to changing conditions, on the one hand, and to prove their legitimacy, on the other. Evolutive interpretation is one option to meet these challenges without formal amendments. This option is especially resorted to in the European context (ECHR, EU). Another option is to interpret treaties according to their parties‘ “subsequent practice” – a means of interpretation which is enshrined in the Vienna Convention on the Law of Treaties (VCLT). Based on empirical-comparative studies, this project aims at gaining insight into possibilities and limits when interpreting international treaties in the light of “subsequent practice”. Thus, it is intended to distinguish different forms of “subsequent practice”, to assess them legally, to analyse their interrelation with other international requirements and to identify limits for taking account of “subsequent practice”, which may result from ideas of legal certainty and legitimacy.