Sustainability is widely recognized as a key objective of urban development in the twenty-first century. But how is it realized in practice? What are its historical origins and its theoretical underpinnings? How does it connect to, or inform, related movements which seek to create more liveable cities, achieve more equitable development, or reduce greenhouse-gas emissions? What are the best examples of sustainable cities in practice, and how did they get there? Can megacities really be sustainable? And how do sustainability aspirations compare between developed and developing nations?