Constructivists often argue that International Organizations (IOs) diffuse norms throughout the international system. This article asks the question: if IOs promote and diffuse specific norms within world politics, where do these norms come from? In particular, this analysis seeks to formulate how IOs' identities emerge in issue areas where rationalist theories give limited explanation, such as the environment. This article posits that IOs interact with and consume norms from non-state actors such as transnational advocacy networks, a process overlooked by the constructivist analysis of institutions. This is examined through a case study of the World Bank's environmental identity where transnational advocacy networks played an important role in the Bank's shift towards sustainable development, through processes characterized here as direct and indirect socialization. This article demonstrates that the Bank's shift was more than instrumental as a result of this interaction, and that constructivists therefore need to examine the role of IOs as norm consumers as well as norm diffusers.