The signing of international treaties is usually considered insignificant for international legal cooperation. Accordingly, International Relations theorists have paid it little attention. We show in this paper how and why treaty signature matters for the ultimate decision to ratify an international treaty. We argue that when multiple well-informed actors publicly sign an international treaty, this can provide a strong signal of issue importance to domestic veto players, and in turn may persuade them to ratify the treaty. We formalize this argument in a two-level signaling game, and test it on a data set of 126 international environ- mental agreements. We find that treaties are more likely to be ratified when their signatories include countries with high levels of general or issue-specific knowledge.