Cities are today at the root of our ecological crisis because cities are where humans consume the environment. With global networks channelling resources, cities today consume the environment with staggering efficiency. As a result the climate is being destabilized, with possibly catastrophic long term effects. In this paper we argue that 'sustainability' has been stripped of meaning by overuse and misuse in the interests of sustaining nothing but the status quo. We explore a rationale for limiting the term 'sustainability' to the project of sustaining the planetary ecology. It must be recognised that, whilst the market and democratic politics can, if properly adjusted by planning, deliver some approximation of efficient allocation and social justice, there is no such institution that can limit the scale of the economy. Thinking out such an institution and bringing it into being is perhaps the single greatest political challenge of the twenty first century. For the rest, almost all our human institutions need to be transformed if ecological sustainability is to be assured.