The law of treaties is one of the branches of international law whose roots go back furthest in time. With the emergence of political communities came the need for them to deal with each other, to settle questions in dispute without having to go to war, to arrange the consequences of success or failure after a war had been fought, to strike alliances, organise matters of trade, settle territorial limits to their power, and so on. For such matters they needed from early times some accepted rules covering two matters, the sending of envoys and the making of agreements. Both have remained central to the conduct of what we now call international relations.

Over centuries, the rules and practices governing those agreements have evolved into the modern law of treaties. The evolutionary process is a continuing one. A book on the law of treaties written at the end of the nineteenth century is recognisably about the same subject as its equivalent written today. Yet, while the general body of the law remains broadly stable, times change and bring with them changes in the law. International organisations have emerged as significant actors in the treaty-making process; multilateral treaties are nowadays concluded more frequently, and have more parties, than used to be the case – a reflection of the enormous increase in the number of states during the course of the present century – and there have been great technological changes, especially in communications, which have noticeably affected the process by which treaties are negotiated and concluded.