Global governance is no longer a matter of state cooperation or bureaucratic politics. Since the end of the cold war, advocacy groups have proliferated and enjoyed increasing access to global governance institutions such as the European Union, World Trade Organization, and the United Nations climate conferences. This special issue seeks to push theories of interest groups and international non-governmental organizations forward. We argue that the advocacy group effects on global governance institutions are best understood by examining how groups use and shape domestic and global political opportunity structures. The individual articles examine how, when, and why domestic and global political opportunity structures shape advocacy group effects in global governance, across global institutions, levels of government, advocacy organizations, issue areas, and over time. As special interests are becoming increasingly involved in global governance, we need to better understand how advocacy organizations may impact global public goods provision.