The urban population of the world now exceeds the rural population and continues to grow. Towns and cities dominate the demand for energy, transport, water and waste treatment. There is a pressing need to find integrated solutions for energy, transport and infrastructure, if towns and cities are to become truly sustainable. Urban systems are more easily optimised if built on greenfield sites and this is the focus of many efforts around the world; but we have chosen to focus on the harder problem, which is also the biggest problem. Our aim is to find engineering solutions for the vast numbers of existing towns and cities around the world, where rebuilding from scratch is not an option. Solutions that will enable them to become self-sufficient in energy and water while at same time providing their citizens with cleaner air, greater mobility and flexibility.