As the activities of the degrowth community emphasize, radical sustainability transitions require both a clash with business-as-usual and the development of experiments that can demonstrate better ways of living and organizing the society. In these endeavours, both practical and conceptual activities are needed, including the development of a new economics. Mainstream economics constitutes a serious barrier to sustainability transitions in many ways: by analysing economic conditions in misleading ways, by providing ideological support for inequality, and by promoting insufficient or even counterproductive measures. Since ideas from mainstream economics permeate media debate and influence policies heavily, there is a strong need for a new economics that is supportive of socially just sustainability transitions. Ecological economics is well positioned to ensure that a biophysical perspective becomes foundational in the development of such a new economics, but it has to be combined with contributions from other streams of so-called heterodox economics (institutional, evolutionary, marxist, feminist, post-keynesian, etc.) to constitute a strong critical alternative to mainstream. As these communities tend to focus on their particular difference with mainstream, the integration of key elements from these approaches into a new economics should be a high priority.