Without treaties, international law and international relations are difficult to imagine. From the dramatic to the mundane, so much activity today is regulated by treaties. Where war is waged, we argue about the scope of the Geneva Conventions, human rights treaties and of course the UN Charter. When States make peace or draw boundaries, they do so by treaty. Where individuals suffer, international agreements provide us with a language and a benchmark to characterise atrocities as ‘crimes’ or ‘human rights violations’. When we are about to board a flight, we make use, usually unconsciously, of international rules on standardised passports derived from treaties, and we benefit from international conventions on civil aviation. Some treaties reflect the international community’s hope for a more just world order, others entrench grave injustices. Treaties are ubiquitous: since the end of World War I, around 56,500 have been registered with the United Nations and its predecessor, as envisaged in Article 18 of the League’s Covenant and Article 102 of the UN Charter. This figure, however, does not reflect the total number of treaties concluded: it includes neither oral agreements nor treaties between non-UN members; and of course it does not capture the considerable number of treaties that, contrary to Article 102, have not been registered. International law is difficult to imagine without treaties indeed.