Answers to the question “What is a treaty?” are at once deceptively simple and dizzyingly complex, depending on who is asking it and for what reasons. The meaning of a treaty in the ancient world was evidently not the same as its significance in the contemporary era of digital diplomacy; the conceptions of a treaty among Indigenous peoples in North America, South and Southeast Asia, Africa, or Oceania were not the same as the understanding of treaties within the Eurocentric system of international law; the significance of a treaty also differs in a settler colonial society where it forms the basis of an intercultural social contract—for example, in Aetearoa/New Zealand where the Treaty of Waitangi (1840) fulfills that function—from those polities where treaties are primarily objects of public international law; a treaty’s import will not be the same for a student of material culture as for a practising diplomat, nor is it obvious how legal theory might integrate diplomatic documents into the broader social contract tradition. Such distinctions and discussions should be a productive source of intercultural comparison as well as a spur to conceptual innovation. Only a dialogue among disciplines--history, international relations, political science, law, and art history among them--can tackle such proliferating questions. It is especially appropriate to convene such a meeting in 2019, the fiftieth anniversary of the Vienna Convention on the Law of Treaties of 1969.