The impact of cities on the environment increasingly dominates the debate on sustainability. Most global and regional environmental problems originate in cities. Cities concentrate increasing numbers of people and human activities; thus, they import increasing amounts of natural resources and export vast quantities of emissions and waste. Urbanization also entails major changes in the way people use natural resources. While it accelerates the transition from traditional to modern fuels, it also intensifies the use of energy and its environmental impacts. Indeed, a nation's levels of energy use and greenhouse emissions are both positively correlated with its urbanization level (Jones 1991; Hosier et al. 1993; Parikh and Shukla 1995). On the other hand, cities provide major opportunities to achieve economies of scale and use natural resources more efficiently. Compact urban settlements, for example, are generally more energy efficient than dispersed ones(Owens 1986; Newman and Kenworthy 1989, 1990; Lowe 1991; Gilbert1992). Thus, the way cities are designed and managed can be crucial to sustainability.