Although not explicitly mentioned in Art. 51 of the UN Charter, there is a consensus that self-defence is inter alia restricted by proportionality. However, the content and scope of this so-called jus ad bellum proportionality are still controversial in international legal scholarship and practice. This book concretises the criterion with the help of the rules of interpretation of the Vienna Convention on the Law of Treaties, taking into account the extensive application of the proportionality requirement by states and UN institutions as subsequent practice pursuant to Art. 32 VCLT. As practice indicates, this book finally the relevance of the rules of international humanitarian law in the assessment of jus ad bellum proportionality.