This chapter challenges traditional perspectives on questions of sustainability and the future of European welfare states. In recent decades, against the backdrop of population ageing and global economic turbulence, efforts to reform European welfare states have centred on preventing a too heavy financial burden on current and future government budgets. However, faced with an increasingly apparent climate crisis, issues of ecological sustainability place themselves on the agenda of all public policy domains, including social policy. We argue that climate change forces us to rethink how we conceptualize and empirically assess the sustainability of mature European welfare states. The chapter presents the rationale and structure of this edited volume and reviews the literature on 'eco-social' policy. The book raises the question of what advanced European welfare states should aspire to sustain, for whom and, once we have agreed on the aspirations, how advanced European welfare states might work towards such goals.