The ecological sustainability of cities can be measured, at the physical level, by the total flow of energy, water, food, mineral and other resources through the urban system, the ecological impacts involved in the generation of those resources, the manner in which they are used, and the manner in which the outputs are treated.

As cities seek to make their energy, water, biological and materials sub-systems more sustainable, the degree to which the intensification of urban development is supportive of these aims will become clearer.