This paper develops an empirical framework for analyzing the timing of international treaties. A treaty is modeled as a dynamic game among governments that decide on participation in every period. The net benefit of treaty membership increases over time. Spillovers among members and non-members accelerate or delay treaty formation by transforming participation into a strategic complement or substitute, respectively. The predictions of the model inform the estimation of the structural parameters, based on a cross section of treaty ratification dates. With this approach, I estimate the sign and magnitude of strategic interaction in the ratification of the Montreal Protocol, in the formation of Europe’s preferential trade agreements, and in the growth of Germany’s network of bilateral investment treaties. Through a series of counterfactual experiments, I explore different mechanisms that give rise to strategic interaction in the formation of these treaties.