Abstract:The problem of legitimacy is one of the oldest in political theory. What gives some people the right to govern others? Conversely, apart from fear of punishment, why should those subject to authority obey? These questions have been central preoccupations of philosophers since the time of Plato. Traditionally, the weakness of international institutions made legitimacy a less pressing issue in international law than in domestic law. But as international institutions develop with the power to bind non-consenting states, this has begun to prompt concern about the problem of legitimacy. In recent years, legitimacy has begun to emerge as an issue not only in international law generally, but in international environmental law more specifically. The growing severity and complexity of international environmental problems has increased the need for institutions with greater legislative, administrative and adjudicatory authority. This article surveys the various potential bases of legitimacy of international environmental law, including state consent, democratic decisionmaking, participation, and expertise. It argues that international environmental law faces a dilemma. On the one hand, effectively addressing problems will require more authoritative systems of international governance, which do not depend on consensus among states. But, without a firmer basis of legitimacy, states will be unwilling to entrust international environmental institutions with the necessary decision-making authority.