Along with concerns over the effectiveness of earth system governance, ways of enhancing its accountability and legitimacy are increasingly coming to the fore in both scholarly debate and political practice. Concerns over accountability and legitimacy pertain to all levels of governance, from the local to the global, and cover the spectrum of public and private governance arrangements. This conceptual article elaborates on the sources, mechanisms and reform options relating to more accountable, legitimate and democratic earth system governance. We proceed in four steps. First, we conceptualize accountability and legitimacy in earth system governance. Second, we place questions of accountability and legitimacy within the larger context of earth system transformation, which, we argue, poses special challenges to the pursuit of accountability and legitimacy. Third, drawing on the contributions to this special section, we analyze different sources and mechanisms of accountability and legitimacy and their effects on the democratic potential and effectiveness of governance. Fourth, in concluding, we outline reform options that may help alleviate persisting deficits in the democratic potential of earth system governance. © 2011 Elsevier B.V.