Subsequent practice is a means of treaty interpretation codified in Article 31(3)(b) of the Vienna Convention on the Law of Treaties (VCLT, 1969), which directs interpreters to take into account, together with the context, any subsequent practice in the application of the treaty which establishes the agreement of the parties regarding its interpretation.
To qualify, the practice must be (i) in the application of the treaty, (ii) attributable to the parties (or at least accepted by them), and (iii) sufficiently consistent and concordant to demonstrate a shared understanding. One-off statements, unilateral declarations, or contested behaviour generally do not meet the threshold. Conduct by a single party may still be relevant under VCLT Article 32 as a supplementary means of interpretation, even if it does not bind the others.
The International Law Commission's 2018 Draft Conclusions on Subsequent Agreements and Subsequent Practice in Relation to the Interpretation of Treaties, led by Special Rapporteur Georg Nolte, refined the doctrine. The Conclusions distinguish subsequent agreements (Art. 31(3)(a)) from subsequent practice (Art. 31(3)(b)), and recognise a broader category of practice by one or more parties that is relevant but non-determinative.
Subsequent practice can clarify ambiguous terms, narrow or expand obligations, and in rare cases support an evolutionary interpretation. It cannot, however, amend a treaty against its clear terms; the line between interpretation and modification is contested, and the ILC ultimately declined to endorse modification by practice.
For practitioners, identifying subsequent practice involves examining diplomatic notes, voting records in treaty bodies, conference resolutions, domestic implementing measures, and consistent enforcement patterns. Atlas users analysing disputes over treaty meaning should look for patterns across parties rather than isolated acts.
Example
In the WTO Appellate Body's US–Gambling dispute (2005), panels examined member states' subsequent practice under the GATS to interpret the scope of 'recreational services' commitments.