ABSTRACT
In the ten-thousand-year history of cities, there have only been two significant transitions which have shaped urban form: the transition to agriculture, and the transition to machine age. A third transition is coming, brought about by the declining availability of cheap fossil-fuel energy. In the past century, it has been cheap energy which has permitted cities to expand logarithmically by using low-cost, high-quality energy to support every aspect of urban life. If the city is to survive, it will have to be designed for sustainability, balancing its resource use with continuously available supplies.
