The law of treaties

![CDATA[Before the 20th century, most rules of international law were in the form of customary international law. Since then, the increased complexity of international relations and rapid international development have led to a substantial growth in the number and diversity of treaties. Article 38(1)(a) of the Statute of the International Court of Justice (‘ICJ Statute’) recognises treaties as a (material) source of international law by referring to ‘international conventions, whether general or particular, establishing rules expressly recognized by the contesting states’. Treaties now regulate trade, communications, environmental protection, military cooperation and defence, and human rights, to name but a few of the myriad topics. International environmental law, for example, is almost entirely governed by treaties, and international trade, investment and communications ‘are unimaginable without treaties’. The main rules in the law of treaties are contained in the 1969 Vienna Convention on the Law of Treaties (‘VCLT’), which governs treaty relations between states and is the focus of this chapter.]]