This chapter reviews some of the tenets of constructivist International Relations theory by illustrating the social ontology that lies at the root of constructivist notions of systemic constitution. The agency accorded to Non-governmental organizations (NGOs) as private, non-state actors therefore depends on the contemporaneous structure of the international or global system within which they operate. It revisits the constitution of prior systems in order to demonstrate how the institutionalization of the sovereign-territorial states-system not only led to the separation of “public” and “private” realms but paved the way for the very possibility of “non-governmental” organizations. The chapter evaluates how NGOs have acquired diverse discursive and operational roles, allowing them to play a meaningful role in contemporary World Politics. It analyzes critical constructivist perspectives, exposing some limitations inherent to “liberal” approaches. The chapter suggests that the communicative turn based on a logic of argumentation could offer a path to reconciling this cleavage through an “agonistic” constructivist synthesis.