Nongovernmental organizations (NGOs) are increasingly important participants in international environmental institutions. NGOs have been formally—but not fully—incorporated into what were previously “states-only” activities. This article surveys these new participatory roles and offers an analytical framework for understanding the pattern, terms, and significance, for international theory, of NGO inclusion. NGOs are distinctive entities with important skills and resources to deploy in the process of international environmental cooperation. Rather than undermining state sovereignty, active NGO participation enhances the abilities of states to regulate globally. The empirical pattern of NGO participation has been structured across time and functional areas to reap these gains. Recent evidence from the restructuring of the World Bank's Environment Facility is used to test these claims. That NGOs are now more pervasive in international environmental institutions illustrates the expansion, not the retreat, of the state in addressing global environmental problems.