This chapter aims to share the ICRC’s experience and insights in applying the treaty interpretation methodology set out in the Vienna Convention on the Law of Treaties to the interpretation of the Geneva Conventions. It draws on the work accomplished in updating the first three Commentaries on the Geneva Conventions, focusing on the Third Geneva Convention. It explains how the rules for treaty interpretation have been applied in this process. It looks in particular at the requirement that an interpretation be carried out in good faith, the concepts of the “ordinary meaning of terms” and the “object and purpose” of a Convention. It further examines the impact of subsequent developments in practice and law on the interpretation of the Conventions and surveys the supplementary means of interpretation set out in the Vienna Convention. Finally, it examines how the Commentaries have drawn a distinction between the law as it stands today (lex lata) and the law as it should be (de lege ferenda).