We are entering a new urban era in which the ecology of the planet as a whole is increasingly infl uenced by human activities (Turner et al. 1990 Ellis 2011; Steffen et al. 2011a, b; Folke et al. 2011). Cities have become a central nexus of the relationship between people and nature, both as crucial centres of demand of ecosystem services, and as sources of environmental impacts. Approximately 60 % of the urban land present in 2030 is forecast to be built in the period 2000–2030 (Chap. 21). Urbanization therefore presents challenges but also opportunities. In the next two to three decades, we have unprecedented chances to vastly improve global sustainability through designing systems for increased resource effi ciency, as well as through exploring how cities can be responsible stewards of biodiversity and ecosystem services, both within and beyond city boundaries.