Cities form a vast glob­al net­work con­nect­ed by flows of ener­gy, food, infor­ma­tion. This glob­al net­work is the chal­lenge of the 21st cen­tu­ry. How do we make more sus­tain­able cities, with small­er eco­log­i­cal foot­prints and more equi­table human well­be­ing?