International environmental policy (IEP) has the potential to influence the quality of life or the possibility of life, for virtually every organism in the world. Inputs from states and non-state actors shape the substance of such policy. Recently, numerous researchers have studied the role of non-governmental organizations (NGOs) in the formation of IEP. State-centric approaches claim little to no influence on the part of NGOs, while other researchers insist that NGOs have an important role in policy formation. In this paper, a detailed framework is used, which identifies the methods of influence available to NGOs and a variety of factors that may enable or constrain their efficacy. Numerous interviews were conducted with NGO representatives to help inform the frameworks emphases. The framework is employed to test the claims of various commentators by ascertaining the extent of NGO policy influence at two recent international conferences: the 1999 International Joint Commission Biennial Conference, and the 2001 Conference of the Parties, Part 11, to the United Nations Framework Convention on Climate Change. It is found that NGO policy influence at both conferences ranged from none to slight. The best method of influence available to NGOs is found to be the shaping of public opinion in powerful states, so that it is agreeable to NGO demands. The findings bolster state-centric claims, which place contemporary international political power largely in the hands of governments. Additionally, the findings acknowledge the power of public opinion in forming the preferences of governments. Lastly, public opinion in more powerful states is found to be more significant, in terms of policy influence, then its counterpart in less powerful states.