Chinese eco-cities can be seen as the largest experiment in urban sustainability ever, and the various pilot projects currently under way will provide many innovations in green technology and valid lessons on sustainable urban development. Given the scale and speed of urbanisation, however, these innovations will come too late for many cities and regions and a huge amount of unsustainable urban development will then have to be retrofitted. In the future it will be more critical to apply the eco-city lessons to the existing urban areas than to keep developing new urban areas, even if more sustainable. Quoting the author: “Eco-cities as isolated areas are the first step in developing urban sustainability, with the development of existing urban districts as eco-cities as the next logical step”. The article also explains the advantages of creating eco-districts and advances the vision of Beijing Core Area as an Eco-District.