The theory of representative democracy is based on the assumption that representatives can be held accountable by their voters. Additionally, the theory of democratic elitism introduced the idea of horizontal accountability, assuming that competing elites serve as checks and balances for one another, thereby making up for deficits in vertical accountability. However, globalization has given rise to an ever-increasing number of transnational institutions as well as multinational corporations and NGOs for which both theoretical models of accountability do no longer hold. Lacking both a territorial basis and a public mandate, the ascendancy of these actors poses theoretical, empirical and normative questions regarding their accountability to national or international audiences. The paper will discuss these theoretical questions and develop a research agenda for studying them empirically.