A central theme in any discussion of global change is diffusion. Stories about diffusion are explanations is diffusion. Storied about diffusion are explanations of how particular organizational models, practices, norms, strategies, or beliefs spread within a population. As constructivists have considered matters of diffusion, they have highlighted two important issues. One is institutional isomorphism, which observes how actors and institutions that are subjected to the same environment will frequently acquire identical forms. In other words, if once there was a diversity of models within the population, over time that diversity yields to conformity and convergence around a single model. There used to be various ways to organize state structures, economic activity, education systems, free trade agreements, and on and on. But now the world is organized around the nation-state, modern states increasingly adopt democratic forms of governance, economies are increasingly organized around markets, most international organization now have a multilateral form, and a growing number of regional trade agreements now have labour and environmental side-agreements. This convergence might be driven by the fact that the world now knows that some institutions are just far superior to others. Yet there is the additional possibility that this convergence is driven not by the search for efficiency but rather by the desire for acceptance, symbolic legitimacy, and status. The other issue in the origins, rise, and widespread acceptance of various international norms. Norms of humanitarianism, citizenship, military intervention, human rights, trade arms control, and the environment have helped to organize world politics, define the purpose of state activities, and have shaped what counts as acceptable behaviour. The constructivist concern with diffusion then, touches centrally on global change because of the interest in the world in motion and transformation