This essay, forthcoming in the Oxford Guide to Treaties, surveys the role of NGOs in treatymaking. It asks four key questions. First, what roles do NGOs play today in treaty processes, and how have these roles changed? Second, what explains the increased prominence of NGOs? Third, are NGOs a salutary addition to treatymaking or illegitimate special interests? And finally, Four, what is the broader significance of NGO activity for international law and international order? NGOs are clearly important players in treatymaking today. But their roles remain, to a large degree, circumscribed and controlled by states. Indeed, NGO participation is frequently useful to governments and the rules and practices regulating their participation in treaties reflect this often symbiotic relationship. And at the most fundamental level, the presence of NGOs in contemporary treatymaking and implementation- which for brevity I will refer to as treaty processes- is a sign of the expansion of the domain of treatymaking. As international law has increasingly expanded its substantive ambit, it has come to govern more and more of what has traditionally been thought to fall within the sphere of national law. In turn, the process of international lawmaking has come to resemble the process of national lawmaking. Like lobbyists in national capitals, NGOs tell us where governing power is; increasingly, that power rests at the international level.