Article 18 of the Vienna Convention on the Law of Treaties (VCLT) plays an indispensable role in promoting stable relations amongst States by obliging them to refrain from acts which would defeat the object and purpose of pending treaties. However, for more than 50 years since its adoption, Article 18 has lingered in a state of legal uncertainty. This book offers a complete guide to the precise scope and content of Article 18 VCLT by analysing its particular elements. Of relevance to scholars, practitioners, and postgraduate students of international law, it applies Article 18 VCLT to contemporary events in international law. It showcases the vitality and direct relevance of the provision in today’s international legal order, while offering concrete arguments for its effective application.