This chapter examines the principles and rules of the international law of treaties as reflected in the 1969 Vienna Convention on the Law of Treaties (VCLT). It discusses the treaty as a legal concept and provides an overview of the regulation of who can conclude treaties, how consent to be bound by a treaty is expressed, the rules on entry into force, treaty reservations, the interpretation of treaties, amendments and modifications, the invalidity of treaties and the termination of and withdrawal from treaties. The VCLT is meant to be applied to all types of written treaties and it therefore governs treaties as diverse as a bilateral agreement to construct infrastructure as well as a multilateral document such as the UN Charter. In practice, however, the concrete application of the Convention may differ depending on the type of treaties.