International law sometimes develops through great lawmaking moments, such as treaty adoption. It also develops through the quotidian interpretations that develop the meaning of those laws over time. The paper argues that interpretive interactions at the implementation stage and at later points in the life of a treaty or customary norm are an important form of “post hoc” international lawmaking, both as a doctrinal and practical matter. Descriptively, these interactions attract interest and lobbying by non-state actors, including business groups. Theoretically, this process-based account supports critical and constructivist views of interpretation, and extends the literature on pre-treaty norm entrepreneurship. Normatively, interpretive contests matter because they implicate rules of law values. Looking to the future, because the 21st century has not featured many big international lawmaking moments, the small ones may matter more: older regimes may attract more interpretive attention.