This thesis addresses a recurrent question of our time – whether democracy can secure environmental sustainability – by drawing on literatures in the normative theory of democracy, social choice theory and environmental politics. I propose a basic, yet substantial organising principle, the ‘dilemma of green democracy’, which maps out the possibility of realising green outcomes under democratic constraints. Interdisciplinary ideas from neighbouring disciplines are also imported for the purpose of studying the design of good environmental-democratic institutions. The analytical framework is an integrated one, comprising formal choice theory and normative democratic theory. The first part of the thesis focuses on the possibility of environmentaldemocratic institutions. Chapter 1 introduces the dilemma of green democracy – a conflict between three plausible desiderata for environmental democracy – and suggests several proposals for avoiding the dilemma. It concludes that, as long as the dilemma is resolved, it is logically possible to construct environmental-democratic institutions. Chapters 2, 3 and 4 assess the desirability of the different proposals in terms of procedure and outcome. The general conclusion is that whether these proposals are desirable depends on a number of conditions and/or contextual factors. The second part of the thesis examines the substantive issues in designing environmental-democratic institutions. Chapter 5 discusses how the discursive dilemma in social choice theory and the normative ends of deliberation constrain the inputs of such institutions. Chapter 6 demonstrates how the concept of distributed cognition, drawn from cognitive/computer science, reconciles the tension between technocracy and democracy. Chapter 7 suggests how the theory of cognitive dissonance, drawn from psychology, challenges the epistemic performance of practicable (environmental-) deliberative-democratic institutions. The overall conclusion is two-fold. First, democracy can, at least in principle, secure environmental sustainability, provided that the dilemma of green democracy is resolved. Second, interdisciplinary ideas are useful for designing good democratic institutions for collective environmental decision-making. This conclusion has implications not only for intellectual enquiry, but also for institutional design in practice.