Following a host of high-profile scandals, the political influence of platform companies (the global corporations that that operate online ‘platforms’ such as Facebook, WhatsApp, YouTube, and many other online services) is slowly being re-evaluated. Amidst growing calls to regulate these companies and make them more democratically accountable, and a host of policy interventions that are actively being pursued in Europe and beyond, a better understanding of how platform practices, policies, and affordances (in effect, how platforms govern) interact with the external political forces trying to shape those practices and policies is needed. Building on digital media and communication scholarship as well as governance literature from political science and international relations, the aim of this article is to map an interdisciplinary research agenda for platform governance, a concept intended to capture the layers of governance relationships structuring interactions between key parties in today’s platform society, including platform companies, users, advertisers, governments, and other political actors.