This introductory chapter sets the scene for the Research Handbook. It retraces how cities gradually develop into internationally relevant actors, how this development has been first traced in other disciplines and how slow the scholarship of international law has been catching up with this development. This picture has changed in the last ten to fifteen years and the chapter contains a state of the art overview of the extant literature in the growing field of international law research on cities as actors in international law, forming transnational networks and being impacted for example by normative expectations of good urban governance. It articulates how the turn of the city to the international also finds its limits in international law and institutions. The chapter argues that it is time to take a further step in the production of international law scholarship towards better understanding how international law is transformed through the growing role of cities. It combines this call with an introduction to the themes of individual sections and contributions of the Handbook.