This paper examines the role and influence of non-state actors in global environmental politics. It draws on the theoretical framework of multilevel governance, emphasising the influence of non-state actors at various policy- making levels. Empirically, we assess and compare the following cases: multilateral environmental negotiations (the climate change negotiations and the International Whaling Commission – IWC), the EU Emissions Trading System (ETS), and non-state environmental and social certification programs The comparison of the climate change negotiations and the IWC reveals that variation in domestic source of influence is crucial for explaining the different influence environmental NGOs have at the international level. With EU emission trading, a comparison of the 2003 and 2008 Emissions Trading Directives reveals that whereas the 2003 outcome was mainly in line with industry interests, the 2008 outcome was more in line with the positions of environmental NGOs. We examine whether this change was a result of NGO influence or other factors, such as the (changing) role of the member states and the EU institutions (European Commission and Parliament). The last empirical section of the paper examines the emergence of environmental and social certification programs created with little or no participation from states. We focus in particular on forest and fisheries certification, while also covering certification initiatives in other sectors and industries. These multi-stakeholder programs are interesting to examine because NGOs and business have agreed on rules and mechanisms to enforce them and collaborate permanently in multi-stakeholder governance bodies. The comparison across cases reveals that there is significant variation in both the role and influence of non-state actors in multilateral, EU-level and private governance programs. Careful attention to this variation is crucial for advancing our understanding of how and under what conditions non-state actors influence policy outcomes.