A multitude of scholarly writings in many languages concern the Vienna Convention on the Law of Treaties (VCLT). Nevertheless, uncertainties and difficulties can be still identified when trying to understand whether and to what extent the VCLT is still central in international law and if it fits into the current international legal scenario. Such uncertainties and difficulties have resulted in the writing of this book. Hence, the chapters here collected aim at untangling the yarns of some open issues and at filling some gaps in order to ultimately establish whether at present the VCLT continues to have a role in international law. The main perspective is that of public international law. However, some room is reserved to problems stemming from the relationship between the VCLT and EU Law. The VCLT is also observed through the lens of private international law.