While most scholars agree that NGOs make a difference in global environmental politics, there has been little systematic work that looks at the actual influence NGOs have on policy outcomes. This paper looks to shed some new light on the question of NGO effectiveness through an evaluation of the role played by NGOs in climate negotiations. We begin with a brief sketch of different kinds of green NGOs, along with a review of the sorts of strategies and resources they employ. Next, we look to gauge the influence that NGOs have had on recent rounds of negotiations to do with compliance, flexibility mechanisms, and appropriate crediting rules for sinks. Our analysis is based on detailed interviews with members of some of the most prominent environmental NGOs involved in climate work. Finally, we suggest, based on our findings, some means by which NGOs may look to extend their influence in the development of the climate regime. Our analysis points to the crucial need for further “insider” capacity—that is, NGOs are likely to have the most far-reaching influence on future climate negotiations if they foster ways to work closely and collaboratively with key negotiators and governments.