The academic discipline of International Relations (IR) has long pondered the questions of what it means to act in international politics and who can do so. However, the particular way in which IR has approached the problem of agency has somewhat masked important dynamics in international politics. By approaching the question of agency as an analytical problem that needs to be resolved before engaging with empirical material, IR has failed to see that who can act is often uncertain and contested. This special issue examines the emergence of international agency as an empirical phenomenon. Rather than analysing what given agents do, the contributions study how practices, performances and networks create and transform agency. In this introductory article, we prepare the ground for this distinct approach to studying international politics. We review how IR has addressed the problem of agency, and we discuss three social–theoretical traditions that see agency as an emergent phenomenon: poststructuralism, performance studies and actor-network theory. Finally, we highlight four insights that emerge from the contributions and challenge how IR has traditionally imagined agency.