This paper is a preliminary draft for eventual inclusion in the Handbook of Law and Economics, A. Mitchell Polinsky & Steven Shavell editors. It reviews and synthesizes the work of economists and law and economics scholars in the field of public international law. The bulk of that work has been in the area of international trade, but many of the ideas in the trade literature have implications for other subfields. Recent years have seen a significant increase in research on other topics as well. The paper begins with a general framework for thinking about the positive and normative economics of public international law, and then proceeds to a treatment of specific topics including customary law, strategic alliances and the laws of war, international trade, international investment, international antitrust, human rights law, conflicts of law, and the international commons (fisheries).