International treaties can be notoriously difficult to amend by formal procedures. They must nevertheless be adapted over time to their changing international environment. Subsequent practice – a well-established tool for treaty interpretation – sometimes diverges from treaty provisions to such an extent that it can no longer be said to constitute an act of interpretation or application, but rather becomes, in effect, one of treaty modification. After examining the parameters of the notion of ‘subsequent practice’ and its application in the process of treaty interpretation in light of Articles 31 and 32 of the 1969 Vienna Convention on the Law of Treaties, the present study focuses on its role as a means of modifying treaty provisions. It d...