This chapter focuses on accountability and legitimacy as a dimension of effective earth system governance. It argues that questions of accountability and legitimacy are becoming ever more important, and that all key elements of the problem structure of earth system governance pose particular and often novel challenges for the accountability and legitimacy of governance. It discusses these challenges and outlines political reform options to cope with them. After explaining the concepts of accountability and legitimacy as they apply to earth system governance, the chapter considers some of the challenges posed by the emerging transformation of planetary systems for securing the accountability and legitimacy of governance systems, including high degrees of analytic and normative uncertainty. It then outlines a number of reform options to address such challenges, such as giving primacy to multilateral decision making over alternative approaches, strengthening the representation of civil society organizations in multilateral institutions and intergovernmental organizations, and creating a UN parliamentary assembly of representatives of national parliaments.