Concepts of ecological and environmental democracy seek to reconcile two normative ideals: ensuring environmental sustainability while safeguarding democracy. These ideals are frequently conceived as being in conflict, as democracy is perceived as too slow and cumbersome to deliver the urgent large-scale collective action needed to tackle environmental problems. Theories addressing the democracy-environment nexus can be situated on a spectrum from theories of ecological democracy that are more critical of existing liberal democratic institutions to theories of environmental democracy that call for reforming rather than radically transforming or dismantling those institutions. This article reviews theoretical and empirical scholarship on the democracy-environment nexus. We find continued theoretical and empirical diversity in the field, as well as vibrant debates on democratising global environmental politics, local material practices, and non-human representation. We argue for stronger dialogue between environmental political theory and empirical, policy-oriented research on democracy and sustainability, as well as further exploration of complementarities between ecological and environmental democracy. We identify four main areas of challenge and opportunity for theory and practice: public participation and populism; technocracy and expertise; governance across scales; and ecological rights and limits.