Usual accounts of international law-making and international legal change focus on formal secondary rules. Others include societal and institutional facts. But international law consists of ideas too. Arguably it exists only in minds. To be sure then, the conditions of ideational change codetermine when and how international law is made, unmade, and otherwise changes. This is what this article is after. It first draws a general sketch of international legal change (including its making and unmaking) to then zoom in on its ideational elements, with a narrower focus on market opportunities for ideas. These market opportunities, it is argued, are determined by: paradigm shifts, struggles between competing schools of thought, the formation of distinct epistemic subfields, the core individuals’ different capitals, and changes in beliefs.